bobcotkg Posted February 17, 2012 Share Posted February 17, 2012 Hi Guys,I bought a '92 Superliner with a 3406 Cat, 18 speed Eaton Fuller and rockwell rears. I know kinda a half breed but...Someone has it geared very stupid. Near as I can tell, it has 4.89 gears in the rear. Don't know the trans overdrive, and have no tag. 11-22.5 tires. It will barely do 60mph at 2100. Thinking changing the rears might be the best? Go to like 3.90s? I want the top speed to be 80 to 85mph. I still need a decent low side, because I use the truck in the woods, but enough highway miles I need road speed without running the engine on the carpet all day.From the investigating I've done, the truck had an Eaton Fuller RTX15715 (15 speed?) and 4.56 gears. What would that put it at?Any suggestions?I do have an Autocar log truck that has an 8LL and 3.90s with 22.5s and it seems to be geared very nice for what I'm doing.Thanks all,Bob 1 Quote Link to comment Share on other sites More sharing options...
slpwlker Posted February 17, 2012 Share Posted February 17, 2012 I would go down to 4.11 That should get you upwards of 75 to 80 mph and or put tall rubber on it. I think the 15 would be to long legged but with the same gearing you might be able to get 70 or 75 also just by putting the 15 in and leaving the gearing alone. I got an 18 unknown top gear ratio,on tall rubber with 4.11's that will runn in the mid 90's food for thought 1 Quote Link to comment Share on other sites More sharing options...
David Posted February 17, 2012 Share Posted February 17, 2012 You need to get the numbers off the trans. to be sure. Eaton has about 4 different top gear ratios. Quote Link to comment Share on other sites More sharing options...
Joe Ditchkus Posted February 17, 2012 Share Posted February 17, 2012 RTLO-16618 has final ratio 0.85 RTO-15715 final ratio 0.79 Joe D. Quote Link to comment Share on other sites More sharing options...
lucerne tractor Posted February 17, 2012 Share Posted February 17, 2012 2100 on a 3406, ouch!! Mine had 4:11's and a heavy spec 10 speed, it ran 65mph@1650 if I remember right. That was on 11-24.5 never had to turn it much over 1750 the pulling power is just off idle up to maybe 1500found this on the C15, might help get you where your going The power rating when used in a truck or bus is 435 to 625 horsepower at 2,100 rpm; RV and fire truck ratings are 600 to 625 horsepower at 2,100 rpm. The total torque output is 1,550 to 2,050 foot-pounds at 1,200 rpm. TotalWhen equal to or less than a gross combination weight -- GCW -- of 80,000 pounds, a truck with 1,750 foot-pounds should have an axle ratio of 1,400 rpm at 65 mph. When over 90,000 pounds GCW, a truck should be geared at 1,500 to 1,650 rpm at cruise speed. 1 Quote Link to comment Share on other sites More sharing options...
slpwlker Posted February 17, 2012 Share Posted February 17, 2012 2100 on a 3406, ouch!! Mine had 4:11's and a heavy spec 10 speed, it ran 65mph@1650 if I remember right. That was on 11-24.5 never had to turn it much over 1750 the pulling power is just off idle up to maybe 1500found this on the C15, might help get you where your goingThe power rating when used in a truck or bus is 435 to 625 horsepower at 2,100 rpm; RV and fire truck ratings are 600 to 625 horsepower at 2,100 rpm. The total torque output is 1,550 to 2,050 foot-pounds at 1,200 rpm. TotalWhen equal to or less than a gross combination weight -- GCW -- of 80,000 pounds, a truck with 1,750 foot-pounds should have an axle ratio of 1,400 rpm at 65 mph. When over 90,000 pounds GCW, a truck should be geared at 1,500 to 1,650 rpm at cruise speed.Thats about where mine is at 1550 or so in 18 at 65. Just inside my power range. I try not to lug my 3406E down as my egt's start climbing over 900 degrees Quote Link to comment Share on other sites More sharing options...
bobcotkg Posted February 23, 2012 Author Share Posted February 23, 2012 Well Thanks guys this helps.Heres the story. No tags, can't find any identification on the trans, other than we know its an Eaton. Checked trans for OD ratio by using an electronic tach on damper and parted driveshaft. Comes out to like .2 overdrive to 4.56 gears. (checked rears 4.56 is what it has for sure) I'm thinking I'll put 3.90s in and it will bring it to 70mph, tall rubber should bring it to 76ish. Still less than perfect, but hate to go higher rear ratio because of off highway driving.Any more input?Bob Quote Link to comment Share on other sites More sharing options...
mack mhe9 Posted February 23, 2012 Share Posted February 23, 2012 how tall's the tire from ground to to top 41ish Quote Link to comment Share on other sites More sharing options...
mack mhe9 Posted February 23, 2012 Share Posted February 23, 2012 just reread this, tall 22.5 should be 41 inch tall times 2100 rpm= 86100 then div. by 336 div. by 4.89, div. by 60mph. that is a .87 od witch would be riight, if its got 4.56 init it comes out to .93 od i dont think thats a gear , hope it helps, Quote Link to comment Share on other sites More sharing options...
bobo Posted February 23, 2012 Share Posted February 23, 2012 check this out http://big-rig-road-speed-calculator.blogspot.com/2011/03/semi-truck-road-speed-calculator.html Quote Link to comment Share on other sites More sharing options...
bobo Posted February 23, 2012 Share Posted February 23, 2012 a 3406 and a 3406E are two total different beast! I wouldnt go any lower than 4.11 bobo Quote Link to comment Share on other sites More sharing options...
bobcotkg Posted March 6, 2012 Author Share Posted March 6, 2012 Well guys, heres what I'm gonna do. I'm going with another 18 speed tranny, but with a taller OD. Guy told me that way I wouldn't lose anything on the bottom, but the double over would put me down the road as needed. Think I found a good used trans, and the guy told me he'd sell my old one for me. Maybe I won't have as much in it this way.Thanks for the input,Bob Quote Link to comment Share on other sites More sharing options...
slpwlker Posted March 7, 2012 Share Posted March 7, 2012 Good luck. Hope it works for yeah as intended Quote Link to comment Share on other sites More sharing options...
bobcotkg Posted March 8, 2012 Author Share Posted March 8, 2012 It sounds hopeful anyway! Quote Link to comment Share on other sites More sharing options...
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