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Which Set Of Gears Makes Your Gear Ratio?


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I'm interested in changing gear ratios to some higher gears and I was wanting to know if this is done by changing the top two gears by the driveshaft or the bottom two gears. I seen some gear sets on ebay that were the top two little gears and they were listed as working for 3 different ratios. Someone else told me that the big ring gear and what he called a bull gear that pulled it were all the same and the ratio came from the pinion right behind the yolk and the gear it meshed with. Is this true or is he wrong? Thanks in advance for any info!

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I'm interested in changing gear ratios to some higher gears and I was wanting to know if this is done by changing the top two gears by the driveshaft or the bottom two gears. I seen some gear sets on ebay that were the top two little gears and they were listed as working for 3 different ratios. Someone else told me that the big ring gear and what he called a bull gear that pulled it were all the same and the ratio came from the pinion right behind the yolk and the gear it meshed with. Is this true or is he wrong? Thanks in advance for any info!

The gear ratio is made up of all of the gears. You need to count the teeth of all of the gears and calculate the ratio. For instance if the pinion gear has 16 teeth and the spur gear that it meshes with has 17 teeth, and the spur shaft has 16 teeth and the bull gear that it meshes with has 55 teeth calculate as follows: 17 divided by 16= 1.0625, 55 divided by 16= 3.4375. These numbers are then multiplied: 1.0625 times 3.4375= 3.65234375 or 3.65 final gear ratio

"Mebbe I'm too ugly and stupid to give up!"

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I'm interested in changing gear ratios to some higher gears and I was wanting to know if this is done by changing the top two gears by the driveshaft or the bottom two gears. I seen some gear sets on ebay that were the top two little gears and they were listed as working for 3 different ratios. Someone else told me that the big ring gear and what he called a bull gear that pulled it were all the same and the ratio came from the pinion right behind the yolk and the gear it meshed with. Is this true or is he wrong? Thanks in advance for any info!

This is like one of Paul Harvey's story's, and I too would like to hear the rest of it. Someone somewhere has a chart to tell what will work with what. The place to start is with which or what series differential and which final drive ratio you presently have, and what final drive ratio you are looking for. It's not exactly like choosing a shirt to match your pants, but it is possible to mix gear sets. Keep in mind that higher numbers are lower gears.

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I'm interested in changing gear ratios to some higher gears and I was wanting to know if this is done by changing the top two gears by the driveshaft or the bottom two gears. I seen some gear sets on ebay that were the top two little gears and they were listed as working for 3 different ratios. Someone else told me that the big ring gear and what he called a bull gear that pulled it were all the same and the ratio came from the pinion right behind the yolk and the gear it meshed with. Is this true or is he wrong? Thanks in advance for any info!

In a single axle, the ring gear turns the differential, the pinion gear turns the ring gear. The pinion gear is spun by the drive shaft. The ratio between the ring gear and pinion gear is your rear ratio. The term "bull gear" refers to the larger of two spur gear that are meshed together. So the technically the "bull gear" would refer to the ring gear, not the other way around.

The smaller gears in the differential itself do not affect the ratio in any way. And neither do the gears in a power divider. BUT there are rears called double reduction where the ring gear and differential has a special set of planetary gears between them that further increases the reduction available.

-Thad

What America needs is less bull and more Bulldog!

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The gear ratio is made up of all of the gears. You need to count the teeth of all of the gears and calculate the ratio. For instance if the pinion gear has 16 teeth and the spur gear that it meshes with has 17 teeth, and the spur shaft has 16 teeth and the bull gear that it meshes with has 55 teeth calculate as follows: 17 divided by 16= 1.0625, 55 divided by 16= 3.4375. These numbers are then multiplied: 1.0625 times 3.4375= 3.65234375 or 3.65 final gear ratio

Great info but WAY too much Math for me. LOL

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This is like one of Paul Harvey's story's, and I too would like to hear the rest of it. Someone somewhere has a chart to tell what will work with what. The place to start is with which or what series differential and which final drive ratio you presently have, and what final drive ratio you are looking for. It's not exactly like choosing a shirt to match your pants, but it is possible to mix gear sets. Keep in mind that higher numbers are lower gears.

It is not a big thing to change the final drive ratio by changing only the Bull Gear and cross shaft. But you still need a gear chart to see what Bull Gear and cross shaft will work with the existing ring and pinion. It may be that you could only go deeper, in which case you might change the ring and pinion and leave the Bull Gear and cross shaft alone. Or, to get the final drive ratio that you want, you might have to change them all. In any event you still need to know how to get from where you are, to where you want to go. To do that you need access to a gear chart. Another consideration is how the proposed change will affect the driveability.

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In a single axle, the ring gear turns the differential, the pinion gear turns the ring gear. The pinion gear is spun by the drive shaft. The ratio between the ring gear and pinion gear is your rear ratio. The term "bull gear" refers to the larger of two spur gear that are meshed together. So the technically the "bull gear" would refer to the ring gear, not the other way around.

The smaller gears in the differential itself do not affect the ratio in any way. And neither do the gears in a power divider. BUT there are rears called double reduction where the ring gear and differential has a special set of planetary gears between them that further increases the reduction available.

Hey Thadd,......ya forgot to tell about tha transmission and engine what it turns. This can become confuseing the back ackards termanology. There I go getting excited and can't spell.

mike

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Hey Thadd,......ya forgot to tell about tha transmission and engine what it turns. This can become confuseing the back ackards termanology. There I go getting excited and can't spell.

mike

lol. I was under the impression he was trying to figure out what gears are involved in determining the rear ratio. Dont worry about spelling. I have gotten away with it the better part of my life thanks to Google and spell check. Remember spelling tests in grade school? Well lets just say my parents had to sign one too many papers saying they did in fact review my poor test results :lol:

-Thad

What America needs is less bull and more Bulldog!

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Guest 45LMSWM

We had I think 4 sets of gears for our 80K rears when we put them in our DM800. If memory serves me correctly, the ratio change was done with the first two gears, and the bull gear stayed the same. I know for a fact we didn't have any spare bull gears laying around cause we had to get new ones the two times we broke axle shafts over the last 10 years. If we had them, we would have at least looked to see if they were the same.

We had the extra gears because the truck the 80s were in originally had several different transmission changes over the years, and the previous owner had to play with the ratio in the rears to get the road speed where they wanted it, and we did the same thing when we put them in our DM to get 45 MPH against the governor.

Correct me if I am wrong, but I was under the impression as well that the bull gear was the same, regardless of what the actual overall ratio would be, at least in regards to Mack, Dual reduction, top loaded carriers of the same weight capacity.

-John

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We had I think 4 sets of gears for our 80K rears when we put them in our DM800. If memory serves me correctly, the ratio change was done with the first two gears, and the bull gear stayed the same. I know for a fact we didn't have any spare bull gears laying around cause we had to get new ones the two times we broke axle shafts over the last 10 years. If we had them, we would have at least looked to see if they were the same.

We had the extra gears because the truck the 80s were in originally had several different transmission changes over the years, and the previous owner had to play with the ratio in the rears to get the road speed where they wanted it, and we did the same thing when we put them in our DM to get 45 MPH against the governor.

Correct me if I am wrong, but I was under the impression as well that the bull gear was the same, regardless of what the actual overall ratio would be, at least in regards to Mack, Dual reduction, top loaded carriers of the same weight capacity.

-John

There are a few different bull gears. Regarding the CRD92/93 series and 112/113 (these are the same differentials except that the 112/113 has a coarser spline on the side gears that drive the axle), there are gears with 51, 52, 54, and 55 teeth. What Bollweevil is saying is correct. You need a carrier chart to determine which gears are compatible. By changing the tooth count on one gear it is possible to change the entire overall ratio. The Mack differential (I assumed this is what gumbie was referring to) is a dual reduction carrier. The first set of gears (the bevel pinion gears in the top of the differential) provide the initial reduction, the lower gear set made up of the spur shaft and the bull gear provide the second reduction. If the differential was a planetary differential, say an SR70/80 differential (manufactured by Renault) it would be necessary to also calculate the gear reduction provided by the hub units as well to calculate the overall differential gear ratio. Mike is right that the engine's torque curve as well as the transmissions gear ratio needs to be taken into account to achieve the best fuel economy. On a side note the CRD92/93 and CRD112/113 differentials are being taken out of production and are being replaced by the CRD150/151 series.

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"Mebbe I'm too ugly and stupid to give up!"

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It didnt take long to clear up that question. Today when I was asking around in person I seen one of those gear charts bollweevil speaks of but the man only had one and no copy machine and when I asked to leave with it to go make some copies, " even some extras for him " you would have thought I asked for the key to his saftey deposit box. The reason I want to change goes as follows. I have a nine speed fuller that I have researched on and found out that top gear is .7 something and I have 4.78 rears best I can read the dots on the housing and my truck tacks high and runs slow everywhere I go. Well I want to change to something like a 3.90 ratio and I want to put a 13 back box on my transmission that if my research is correct ends up with almost the same top gear ratio of .7 something but will allow me to split high range gears to keep my pulling power but once i get to crusing speed in top gear, my revs will be a little lower and also let me go faster should the need arise because now it tacks out at about 71 or 72 MPH. Back to the man with the chart, he told me that I could leave my bull gear and the shaft gear alone (( he said I had a 14 - 51 )) and change my ring and pinion to 16 - 17 and this would give me as close as I can get to 3.90 without changing the bullgear and shaft. Using the math above I get 3.86 ratio which is only .04 from a perfect 3.90 I was shooting for. Let me know what ya'll think. Also, does anyone have a gear chart they can post or a link to one that can be printed off? Come on bollweevil, you know you have one somewhere!!!! :D Yall let me know if I'm headed in the right direction. Thanks everyone for all the great info!

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It didnt take long to clear up that question. Today when I was asking around in person I seen one of those gear charts bollweevil speaks of but the man only had one and no copy machine and when I asked to leave with it to go make some copies, " even some extras for him " you would have thought I asked for the key to his saftey deposit box. The reason I want to change goes as follows. I have a nine speed fuller that I have researched on and found out that top gear is .7 something and I have 4.78 rears best I can read the dots on the housing and my truck tacks high and runs slow everywhere I go. Well I want to change to something like a 3.90 ratio and I want to put a 13 back box on my transmission that if my research is correct ends up with almost the same top gear ratio of .7 something but will allow me to split high range gears to keep my pulling power but once i get to crusing speed in top gear, my revs will be a little lower and also let me go faster should the need arise because now it tacks out at about 71 or 72 MPH. Back to the man with the chart, he told me that I could leave my bull gear and the shaft gear alone (( he said I had a 14 - 51 )) and change my ring and pinion to 16 - 17 and this would give me as close as I can get to 3.90 without changing the bullgear and shaft. Using the math above I get 3.86 ratio which is only .04 from a perfect 3.90 I was shooting for. Let me know what ya'll think. Also, does anyone have a gear chart they can post or a link to one that can be printed off? Come on bollweevil, you know you have one somewhere!!!! :D Yall let me know if I'm headed in the right direction. Thanks everyone for all the great info!

It sounds like you're headed in the right direction.

"Mebbe I'm too ugly and stupid to give up!"

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There are several programs available on line, to calculate road speed. A 13 speed overdrive fuller is about a .87. By adding this rear gear box, along with its auxillary drive gear, to the .73 9 over, you compound the overdrive, to create a 13 double over. When you couple this to a .386 final drive, with tall rubber, you could likely set your hair on fire. You will have to re do your math, I woke up with a head ache. James

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To my knowledge, 3.86 is the "fastest" ratio that can be achieved in a CRD92/93 carrier.

Your 9 speed would have a .73:1 high gear ratio, so at 2100 RPM, with 3.86 rears on 11R22.5 rubber that would give you 93 MPH.

Most Eaton Fuller 13 speeds also have a .73:1 high gear ratio, so nothing will be changed by the trans swap except for the ability to split the hi range gears.

Now for the other side of this equation.

Does your truck ever go off road or have to start off heavily loaded on steep hills? If so, the 3.86 rears will be too "high" to give good startability in severe condtions without abusing the clutch. IIRC, a 9 or 13 speed Eaton Fuller would have a low gear ratio in the neighborhood of 13:1. Multiply 13 x 3.86 and the result is an overall reduction of 50.18:1. In my experience over the years, on 11R22.5 rubber, that overall reduction number (trans low gear ratio x rear axle ratio) needs to be at least 60.0:1 in order to achieve good startability when loaded.

With a 9 speed .73:1 high gear, your current 4.78 rears are showing me a top speed of 75 MPH @ 2100 RPM, and with the 9 speed low gear of approx. 13:1, I'm seeing a 62.14:1 overall reduction in low gear.

.

"If You Can't Shift It Smoothly, You Shouldn't Be Driving It"

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There are several programs available on line, to calculate road speed. A 13 speed overdrive fuller is about a .87. By adding this rear gear box, along with its auxillary drive gear, to the .73 9 over, you compound the overdrive, to create a 13 double over. When you couple this to a .386 final drive, with tall rubber, you could likely set your hair on fire. You will have to re do your math, I woke up with a head ache. James

A basic fuller 9 speed i s 1 to 1 or a direct drive transmission. To achieve or create an overdrive from such a unit, Fuller swapped positions of the 4th and 5th speed gears, on both counter shafts. This also necessitated changing 4th and 5th on the main shaft. This was not as simple They had to broach a new head gear, which is now 4th instead of 5th, and a new 4th which is now 5th. Sound complicated? It is necessary to get the teeth count right. This gave us the "up against the dash" high gear, and odd ball shift pattern, which some people have trouble with. To correct some of the confusion Fuller created the RTX shifter plate with extra rails to get the shift pattern back like it was

A 13 over Fuller on the other hand is over driven in the rear box, and consists of a 9 speed direct or 1 to 1 front box and a .87 overdrive rear box. In my time frame, any time someone wanted a double thirteen, a 4.33 final drive was usually the gear of choice. Everybody always wanted tall rubber, and everyone had to have a horse instead of a Shetland pony.

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A basic fuller 9 speed i s 1 to 1 or a direct drive transmission. To achieve or create an overdrive from such a unit, Fuller swapped positions of the 4th and 5th speed gears, on both counter shafts. This also necessitated changing 4th and 5th on the main shaft. This was not as simple They had to broach a new head gear, which is now 4th instead of 5th, and a new 4th which is now 5th. Sound complicated? It is necessary to get the teeth count right. This gave us the "up against the dash" high gear, and odd ball shift pattern, which some people have trouble with. To correct some of the confusion Fuller created the RTX shifter plate with extra rails to get the shift pattern back like it was

A 13 over Fuller on the other hand is over driven in the rear box, and consists of a 9 speed direct or 1 to 1 front box and a .87 overdrive rear box. In my time frame, any time someone wanted a double thirteen, a 4.33 final drive was usually the gear of choice. Everybody always wanted tall rubber, and everyone had to have a horse instead of a Shetland pony.

Good morning James.

I was basing my calculations on a Fuller RTX14609B, which I had in one of my trucks, and on the newer style double O.D. "X" shift pattern 13 speeds.

You seem to agree that the proposed 3.86 ratio is too high.

I think a 4:17 or 4:42 would be more appropriate, unless it's a "toy" truck that never has to pull a heavy load.

.

"If You Can't Shift It Smoothly, You Shouldn't Be Driving It"

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Good morning James.

I was basing my calculations on a Fuller RTX14609B, which I had in one of my trucks, and on the newer style double O.D. "X" shift pattern 13 speeds.

You seem to agree that the proposed 3.86 ratio is too high.

I think a 4:17 or 4:42 would be more appropriate, unless it's a "toy" truck that never has to pull a heavy load.

.

Yep, If you had a horse that could pull it, you could set your hair on fire.

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Good morning James.

I was basing my calculations on a Fuller RTX14609B, which I had in one of my trucks, and on the newer style double O.D. "X" shift pattern 13 speeds.

You seem to agree that the proposed 3.86 ratio is too high.

I think a 4:17 or 4:42 would be more appropriate, unless it's a "toy" truck that never has to pull a heavy load.

.

I can honestly do about anything necessary with any Eaton Fuller transmission, reason being that I have had to. A triple counter shaft Mack is a different story however, because I haven't had to. I really like the 12 speed in my RS, but it seems to be a direct drive. I believe it would actually climb a tree, especially in deep reduction reverse. With tall rubber and a .450 gear about 58 to 60 is the top speed. I think you may have touched on it before, but what would be involved in over driving the 12 speed, and what ratio would you end up with.

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I can honestly do about anything necessary with any Eaton Fuller transmission, reason being that I have had to. A triple counter shaft Mack is a different story however, because I haven't had to. I really like the 12 speed in my RS, but it seems to be a direct drive. I believe it would actually climb a tree, especially in deep reduction reverse. With tall rubber and a .450 gear about 58 to 60 is the top speed. I think you may have touched on it before, but what would be involved in over driving the 12 speed, and what ratio would you end up with.

man, y'all lost me a long time ago! If the 9 speed is already an overdrive transmission and a 13 speed wouldn't help,I think i'd go get some take-out rears with a lower ratio (higher speed),unless it's some off the wall odd ball rear in the first place, and change the whole rear instead of trying to change the gears in it.

huh?..now i've confused myself...hate when that happens

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Producer of poorly photo-chopped pictures since 1999.

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I can honestly do about anything necessary with any Eaton Fuller transmission, reason being that I have had to. A triple counter shaft Mack is a different story however, because I haven't had to. I really like the 12 speed in my RS, but it seems to be a direct drive. I believe it would actually climb a tree, especially in deep reduction reverse. With tall rubber and a .450 gear about 58 to 60 is the top speed. I think you may have touched on it before, but what would be involved in over driving the 12 speed, and what ratio would you end up with.

You need the TRTXL1070B, the overdrive version of the 12 speed.

I'd have to look at some literature to find the exact high gear ratio.

Never tried to convert a TRTXL107 12 speed to a TRTXL 1070, so I don't know if that's possible or not.

.

"If You Can't Shift It Smoothly, You Shouldn't Be Driving It"

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No need to be confused, Tom. A 9 over is over driven in the front box at .73. The 13 over is a 9 speed direct which is over driven in the rear box at .87. When you combine the two you end up over driving an overdrive. You are absolutely correct in thinking that it makes little or no sense. Keep in mind however that at the time this was sort of popular. 9 speed overdrives were rare, if not readily available. Also you could not buy a .336 rear or find a .355 that you could buy. Most used trucks that anyone could afford, were set up with leasing company specs, a 290 or 350 Cummins, with a 9 speed direct and .370 rears. Dependable, but slow. If you were going to make 36 or 37 trips from the east coast to California, and back, during produce season, you needed a horse of a different color. I have run 1500 miles in California, just picking up my return load. 8 or 10 pick ups were not uncommon. A good hot rod transmission then was the 9558LL. Easy to work on, with big spur cut gears, but rough on it's small counter shaft bearings, with no oil cooler. There is better equipment, more readily available now.

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No need to be confused, Tom. A 9 over is over driven in the front box at .73. The 13 over is a 9 speed direct which is over driven in the rear box at .87. When you combine the two you end up over driving an overdrive. You are absolutely correct in thinking that it makes little or no sense. Keep in mind however that at the time this was sort of popular. 9 speed overdrives were rare, if not readily available. Also you could not buy a .336 rear or find a .355 that you could buy. Most used trucks that anyone could afford, were set up with leasing company specs, a 290 or 350 Cummins, with a 9 speed direct and .370 rears. Dependable, but slow. If you were going to make 36 or 37 trips from the east coast to California, and back, during produce season, you needed a horse of a different color. I have run 1500 miles in California, just picking up my return load. 8 or 10 pick ups were not uncommon. A good hot rod transmission then was the 9558LL. Easy to work on, with big spur cut gears, but rough on it's small counter shaft bearings, with no oil cooler. There is better equipment, more readily available now.

H.H.Moore was getting 9 speeds for a while,all direct drives. The 83,84,and 85 K100's I drove all had 9 speeds with 3.70 rears.Then he bought one IH conventional with a 350 cummins that had some odd ball nine speed. Don't know what model it was but it had 4 speeds in the low side and 5 in the high side which was opposite of the ones I drove.

Producer of poorly photo-chopped pictures since 1999.

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We have a really big nice wall chart that shows all the different ring gear/pinion and bullgear combos but cant really post it here as it is huge. On the 92/93 series there are a 51,52,54 and 55 tooth bull gear (big gear in the bottom) and the various ring gear/ pinion combos that go from 3.65 to 9.59 (according to Mack Carrier Service Chart booklet 13-501). I think they did discontinued some ratios years ago but I cant remember. We usually try to just change the ring gear/ pinion (leaving the bull gear alone)to go up or down. Alot of the time just changing the ring gear/pinion is a big jump up or down so to go just 1 step up or down you have to change all the gears. I guess making the big jump up or down then changing your tire size you could get by with it. My little booklet has 66 pages of gear charts covering all 92/93 series to much to post.

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  • 3 months later...

To my knowledge, 3.86 is the "fastest" ratio that can be achieved in a CRD92/93 carrier.

Your 9 speed would have a .73:1 high gear ratio, so at 2100 RPM, with 3.86 rears on 11R22.5 rubber that would give you 93 MPH.

Most Eaton Fuller 13 speeds also have a .73:1 high gear ratio, so nothing will be changed by the trans swap except for the ability to split the hi range gears.

Now for the other side of this equation.

Does your truck ever go off road or have to start off heavily loaded on steep hills? If so, the 3.86 rears will be too "high" to give good startability in severe condtions without abusing the clutch. IIRC, a 9 or 13 speed Eaton Fuller would have a low gear ratio in the neighborhood of 13:1. Multiply 13 x 3.86 and the result is an overall reduction of 50.18:1. In my experience over the years, on 11R22.5 rubber, that overall reduction number (trans low gear ratio x rear axle ratio) needs to be at least 60.0:1 in order to achieve good startability when loaded.

With a 9 speed .73:1 high gear, your current 4.78 rears are showing me a top speed of 75 MPH @ 2100 RPM, and with the 9 speed low gear of approx. 13:1, I'm seeing a 62.14:1 overall reduction in low gear.

.

Hey HK,

I put the back box off a 13 speed on my 9 speed and we changed the top gears in the rear ends to 16-17 to come up with a 3.86 ratio and it all works great. I to was concerned with take off and hurting my NEW clutch with the tall gears seeing how its just a little 400 horse motor in a dump truck usually loaded around 70 to 75 thousand and constantly off road in soft ground. The guy that did the book work for me got the tag number off my transmission and the tag number from the one we got the 13 back box off of and he told me that the .73 high gear was gonna stay the same so I would get the top end difference I wanted with the different rear end gears and he told me the 13 box had a different reduction gear than the 9 which made gears L,1,2,3,4 lower and the lower low range gears would make up for the high rear end gears and still have good takeoff. Then when I got to high range the thumb switch would take care of the tall gaps between the high range gears so it would keep the tach up in those gears as well. I guess he was right because she takes off better than she did before and once im in high gear my rpms are close to 300 less than they were at crusing speed and if you hold it down the truck will get up there real close to 100MPH which is waaay faster than anything or anyone on in or around that truck needs to be going. I might have just got lucky but it all worked out great. I think when we changed the back boxes we also had to use the accessory drive gear from the 13speed but not for sure. They might have been the same. Alot of what I do is based on dumb luck but I always say "I'd rather be lucky than good any day"

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No need to be confused, Tom. A 9 over is over driven in the front box at .73. The 13 over is a 9 speed direct which is over driven in the rear box at .87. When you combine the two you end up over driving an overdrive. You are absolutely correct in thinking that it makes little or no sense. Keep in mind however that at the time this was sort of popular. 9 speed overdrives were rare, if not readily available. Also you could not buy a .336 rear or find a .355 that you could buy. Most used trucks that anyone could afford, were set up with leasing company specs, a 290 or 350 Cummins, with a 9 speed direct and .370 rears. Dependable, but slow. If you were going to make 36 or 37 trips from the east coast to California, and back, during produce season, you needed a horse of a different color. I have run 1500 miles in California, just picking up my return load. 8 or 10 pick ups were not uncommon. A good hot rod transmission then was the 9558LL. Easy to work on, with big spur cut gears, but rough on it's small counter shaft bearings, with no oil cooler. There is better equipment, more readily available now.

My brother just bought a Mack that has a 350 cummins (2200 RPM), direct 10 speed roadranger,3.87 rears,22.5 tall rubber i thought it would at least run 70 mph but the GPS only showed it would run 63.dose that seam right.
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