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Right now I'm running 4.42 in my 97 rd with 400 and 12spd. Problem is now I'm running the highway and I don't like 65mph at 1700rpm. I would like it to be at 70mph at 1700rpm. I would like to run 65mph at 1600ish if not a little lower. Will I achieve this with a 4.17 ? How much bottom end I'm I gonna lose? I'm running 11R 24.5. Also can I run to different ratio rear ends like a front 4.42 and a rear 4.17?

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you cant have different ratios in the same tandem.

assuming you have 11r22.5 tires

3.87 ratio will get you to 70mph at 1750rpm (68.72mph @1700rpm)

 

4.17 at 1700rpm with a 12 speed will put you at 64mph

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Im not trying to be a smart ass here but you do have a 12 sp trans and half gears with a 12 speed! a half gear lower to low? just asking!

your sweet spot will likly be 1400- 1500 the 1600 will be best fuel economy with the half gears you should fine something that fits with the 3:87s!  

Edited by fjh
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Not quiet following. It's a dump truck. They still go off road. I have a rear going so I figure try to kill two birds with one stone. I'm unfamiliar with that ratio because all the macks I ever driven were quads, 6, and 7spd and the rear ratios are up in the 5s so some advice is still needed. Are you saying the 12 doesn't go good with 3.87... what are you saying

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My concern is loosing a lot of pulling power. It pulls well for a little 400. I know I'm gonna lose a little but I don't want to lose whole lot. If it was a cat.. no problem 3.91 here we come. I've ran 120k with that setup but it's a mack. 11liter. Trying to keep balance. 

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Ok.  real quick... Lots to absorb here.... If you have a bad carrier (4:42) Then change both to (4:17), and reset the dipswitches on the speedo. I have the chart on this if you need info. basically, Final drive ratio hopefully OD .76:1 ish... 16 teeth on the output tone wheel, then 4.17: rears, and tire revs/mile.  do you know how to do it the old timey way? I can show you the chart...  by the way. I bet that truck can pull anything the way it's set now?? :) Jojo

Edited by Joey Mack
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Posted (edited)

5th gear is 78. Yes it pulls very well. Have a little timing thrown in there as well. Problem is we( as time goes on) are running more highway. At 65mph its up around 1700rpm ...1800. I want to get those rpm down. I would also like to be topping out around 80mph. It's not necessary but I would like. As I said before...the tires are 11r 24.5. Again. Any help.is appreciated 

Edited by Bliue dm
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ok.. only reason i posted is because I have done many Mack rears and 4.17: 1  and 4.42:1,,, were the most common by far...  Someone who knows more than me will post for you..jojo

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You can park on a clean floor. mark the tire and the floor at 6 O'Clock. roll the tire until the mark hits 6 O'Clock again. mark the floor, measure the distance between the marks and divide it into 5280... thats your revs.. jojo

Edited by Joey Mack
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1 minute ago, Joey Mack said:

ok.. only reason i posted is because I have done many Mack rears and 4.17: 1  and 4.42:1,,, were the most common by far...  Someone who knows more than me will post for you..jojo

If I go 4.17 with 78 final with the 11r 24.5 where would that put me

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2 minutes ago, Joey Mack said:

You can park on a clean floor. mark the tire and the floor at 6 O'Clock. roll the tire until the mark hits 6 O'Clock again. mark the floor, measure the distance between the marks and divide it into 5280... thats your revs.. jojo

Ok. Understood. What is 5280?

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Hell yeah!!! thats a great calculator.... I searched the web, but didnt find that one...jojo

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Posted (edited)

Yes. Seems to be pretty accurate too. I think I'm gonna do 4.17. If it was a bigger motor I wouldn't be so concerned with the loss of tq but I think 3.87 is gonna be a bit much in terms of loss of pulling power. So now that I have the mph figured out let's talk about loss of pulling power. I guess experience is gonna have to answer this questions.

Edited by Bliue dm
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I once had a DM dump put a overdrive 12 speed with 4.17 rears and loved it, was on 22.5 rubber, but the pump was set to 2300 RPM'S would not push it that hard very often, just surprise another truck or two!! it would bury the 80MPH. speedometer.    terry:MackLogo:

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3 hours ago, terry said:

I once had a DM dump put a overdrive 12 speed with 4.17 rears and loved it, was on 22.5 rubber, but the pump was set to 2300 RPM'S would not push it that hard very often, just surprise another truck or two!! it would bury the 80MPH. speedometer.    terry:MackLogo:

Family had 5 70s dm800 673t

One 85 685sx with a 675 tip turbine with a 6spd. It would do 67mph. But for the setup.... pulled like a savage.

 

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