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Rear End Gear Ratio


Alie&Jim's Carrilite

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Currently, our truck has a Rockwell rear-end with a 2.93 gearing. At 68 mph, the engine RPM's are at 1700, which is near the upper rpm limit, and is just out of the best fuel mileage range.

So, would it be possible for the math guru's to calculate what RPM I would have by swapping to a 2.64 gear ratio.

Bonus is that this particular rear end is also a locker and locally available. Haven't seen it and the cover hasn't been pulled to check for damage.

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That would be better, but I was hoping to get under 1500rpm. 1450 is the sweet spot on the M-11"s.

Since I never use the bottom 3 gears, and skip shift / float I'd still drive it like a 5 speed just start with a lower gear.

There is a junkyard near that has a 2009 model out of a front end wreck. $600 and new mounting hardware, fresh oil, and probably have it re-setup "lash". I can do the ford 9" and old gm 10 and 12 bolts, but I think I would need a forklift for this.

Might need to find a 2.5?

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Shallow- How many gears does that Freedomline have? If you have 13, then you can run a higher gearing on the rear. Gives more torque at the low end- 1st and 2nd, but still allows you to have a lower cruising RPM.

Both HeWhoKnowsLittle and I have plain old Rockwell 10sps. No high overdrive like the super 10 etc.

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12 speed seems to be about right. Starts in 3rd and seldom shifts down on the highway except on long steep hills. It stays in 11th until 55mph.

I can shift it manually for any situation that warrants more power.

1700 rpm would have me somewhere around 85 to 90 mph if I remember. I will check next time I go for drive.

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Jim, according to the Meritor web site, the correct ratio at the step you're considering is 2.67, not 2.64. It doesn't make that big of a difference (1549 RPM @ 68 MPH instead of Porky's calculated 1532 RPM, which I verified on an old spreadsheet built for this purpose), but it is slightly different.

 

That said, that may well still be the right ratio, because you might regret trying to go to an even taller gear. The next step higher is, as you said, 2.50, and in your truck that would result in an engine speed drop all the way down to 1451 RPM @ 68 MPH -- right on top of your torque peak, instead of having a 98 RPM cushion -- which could have you downshifting on grades that you now take for granted. A three step move like that may well result in your truck feeling lazier than you'd like, especially with a small-bore engine like the M11.

 

If you know someone at a Cummins distributor, you might ask them what gear ratios Cummins would have recommended/approved with your M11 in combination with the weight you're towing.

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www.roadranger.com for a calculator. NOTE- I swapped my 3.73 (Autoshift) for 3.08's. Went a little too far, 3.23 would have been better W/ my OD ratio & tire size. (But now I top out at 97...............). If you're considering a gear change, not that hard to do but HEAVY. If'n I was to do it again, I'd swap a salvage yard for a factory single of the right ratio, heavier rear & brakes than a tandem singled

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Thank's Phil and Ron. I've thought about finding a factory single to swap in. I just started thinking about this on the way back from the ECR and mentioned this to a fellow camper here in the park. This guy is running dump trucks for a company that is raising the leeve's and needed some scrap steel to rebuild a push bar for one of the trucks, he found the yard that has this rear end in it. Doing a quick search last night resulted with 100's of rear ends that could be swapped.

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[Engine RPM] / [transmission ratio in desired gear] = driveshaft RPM

[Engine RPM] / [transmission ratio in desired gear] / [axle ratio] = axle shaft RPM

[Engine RPM] / [transmission ratio in desired gear] / [axle ratio] / [tire revolutions per mile] * 60 (minutes per hour) = road speed in MPH

 

Example:

1500 / 0.86 (overdrive top gear) / 2.64 / 502 (randomly picked Michelin X Line Energy 1 in 11R22.5) * 60 = 78mph

1300 / 0.86 / 2.64 / 502 * 60 = 68mph

 

Note that regearing your rear axle to get a lower cruise RPM could potentially "force" you to add a shift and rethink which gears you use. Many engines wanted 1500 RPM for a long time, then Cat went to 1350, and now Volvo is walking it down to 1050.

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FYI, in case anybody is wondering, the formula

 

I don't know my transmission ratios . . .

 

I don't know your transmission ratios either, Jim, but I know by your axle ratio that you have a direct drive transmission. While some of the very late model trucks with the newer, ultra low RPM/high fuel economy engines have axle ratios of less than 3.00:1 combined with an overdrive transmission, that idea would have been absurd back when trucks like yours and mine were new.

 

Since Peety offered three other useful formulas, the formula for calculating the answer to the question that you originally asked (Engine RPM at a given speed), and upon which both my spreadsheet and the calculator at roadranger.com is based, is as follows:

 

Speed in MPH X Transmission Ratio X Drive Axle Ratio X Tire Revs per Mile / 60 = Engine RPM @ whatever value you used for "Speed in MPH"

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Don't forget to change your speedometer to match any changes in rear end ratios.

Dave

 

That, strangely, is an engine function, not a truck function. Changes that affect speedometer/odometer functions are controlled by the engine's control module, which then interfaces with the truck's speedometer. It's one of the things to consider before having your ECM scanned and parameters changed.

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Is the little Cummins powerband that much higher than the C12? I like mine to run at 1350-1450 while on the flats for best fuel economy.

 

What is your top trans ratio Phil? I have 3.36's in mine with a .73 top OD netting a final of 2.45 rolling on a 42" tire and would like one more OD when traveling on the flats out of CA when speeds are 70+MPH. I used to leave in 4th unloaded and 3rd loaded, but now one gear lower since swapping out the 3.90's. What I found as the written sweet spot really doesn't apply to me only running below 40K gross combined. I would do 3.08's or 3.21's if I had it to do over again.

 

Jim's situation I guess would be different without OD gears to work with as I don't think I'd want to be driving miles in an under direct gear. Get too tall and the back country roads could end up driving in 8th or 9th.

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Jim does your gen 1 have the 'green band ' on the tach?

I run mine at the very end of mine (high) and I got 8.4 on the way to the ECR and 9.2 back,same RPM same route 3.73 gear.

Loading(wind) made more of a difference then revs,we had way more breeze and much colder on the way there.

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Is the little Cummins powerband that much higher than the C12? I like mine to run at 1350-1450 while on the flats for best fuel economy.

 

What is your top trans ratio Phil? I have 3.36's in mine with a .73 top OD netting a final of 2.45 rolling on a 42" tire and would like one more OD when traveling on the flats out of CA when speeds are 70+MPH. I used to leave in 4th unloaded and 3rd loaded, but now one gear lower since swapping out the 3.90's. What I found as the written sweet spot really doesn't apply to me only running below 40K gross combined. I would do 3.08's or 3.21's if I had it to do over again.

 

Jim's situation I guess would be different without OD gears to work with as I don't think I'd want to be driving miles in an under direct gear. Get too tall and the back country roads could end up driving in 8th or 9th.

 

Our new truck will be running a 3.07 rear and .73 OD just like yours. At 1300 rpm, it should run 68 mph on the nose. On long stretches, 1400 rpm should yield 73 mph which is supposed to be the sweet spot for economy but will be a little faster than I plan on running (just in case a certain enforcement officer with a penchant for Dr Pepper is reading this).

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Is the little Cummins powerband that much higher than the C12? I like mine to run at 1350-1450 while on the flats for best fuel economy.

 

What is your top trans ratio Phil? I have 3.36's in mine with a .73 top OD netting a final of 2.45 rolling on a 42" tire and would like one more OD when traveling on the flats out of CA when speeds are 70+MPH. I used to leave in 4th unloaded and 3rd loaded, but now one gear lower since swapping out the 3.90's. What I found as the written sweet spot really doesn't apply to me only running below 40K gross combined. I would do 3.08's or 3.21's if I had it to do over again.

 

Jim's situation I guess would be different without OD gears to work with as I don't think I'd want to be driving miles in an under direct gear. Get too tall and the back country roads could end up driving in 8th or 9th.

0.73 is a rather tall OD gear...I don't think you want anything taller. It's all relative - you could change the rear axle ratio and get the same target road speeds, and I'd question the sanity of asking the transmission to make the driveshaft spin any faster if the rear axle is just going to make the axle shafts turn the same speeds.

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0.73 is a rather tall OD gear...I don't think you want anything taller. It's all relative - you could change the rear axle ratio and get the same target road speeds, and I'd question the sanity of asking the transmission to make the driveshaft spin any faster if the rear axle is just going to make the axle shafts turn the same speeds.

With the 13spd, we get two OD ratios. So with a 3.08 I could run 50-60 mph in 7th over(direct) which would be perfect of our lower CA speed limits, 8th under (.86) for 60-70mph out of state, and 8th over (.73) for unloaded times where speeds are higher. This way I could actually use the lower gears available with this transmission too.....I never use low or 1st ever. This should theoretically lower both trans and differential temps with less RPM's turned.

 

The only downside I can see is a taller overall reverse gear.

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