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Old 06-08-2011, 08:12 AM   #1
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Figuring out which differential gear ratio is right for you

First, a brief explanation of the drive train. RPM's going in (turns), Gear ratio, Gear reduction and torque multiplication.

Gear ratio is figured out by dividing the driven gear with the driving gear this is usually done by counting the teeth on each gear then divide driven/drive.

In this case all we are concerned with is in coming turns vs out going turns.

a gear reduction is when you have more turns going in than you have coming out.(torque increase)

Overdrive is when you have more turns coming out than you have going in.(torque decrease)


When you hear the term 4.75 to 1 or what ever is over 1.01 to 1 is reduction. what ever the number is that is how many times the torque is multiplied.

If you have 100 ft lbs of torque going in, with a 4.75 reduction you will have 475 ft lbs of outgoing torque at whatever rpm.

High speed (mph) = low torque/low rpm like when you are in O/D

Low speed (mph) = high torque/high or higher rpm like starting off or downshifting to climb a hill.



The transmission is the first gear reduction in the drive line.The second gear reduction (unless you have a bolt on overdrive unit, auxiliary trans or transfer case) is the rear differential.





You need the specs of the transmission gearing ratio that you have. For example I am going to use the Allison AT 545 specs.



To figure out the final drive ratio you have to multiply the transmission ratio and the rear diff ratio.

I will stick with the 4.75 from earlier. So each gear ratio would look like this.

First gear 3.45 x 4.75 = 16.38:1

Second gear 2.25 x 4.75 = 10.68:1

Third gear 1.41 x 4.75 = 6.69:1

Fourth gear 1.00 x 4.75 = 4.75:1

Reverse gear 5.03 x 4.75 = 23.89:1










End of part one
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Old 06-08-2011, 08:17 AM   #2
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Re: Figuring out which differential gear ratio is right for

I will post part 2 tonight. If anyone sees any flaws let me know I will correct them, I am doing this all from the top of my head. I just figured I would put this up because it was a question in one of the sub forums. Hopefully this will help someone understand a little more clearly.
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Old 06-08-2011, 11:43 AM   #3
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Re: Figuring out which differential gear ratio is right for

The final ratio number is equal to the number of turns from the engine or revolutions to turn the rear tire once. Knowing this we can then calculate by the circumference of the tire the vehicles speed and engine rpm.



Next we divide one mile 5280 feet by the circumference of your tire. I will use an 11R22.5 which has a diameter of 42.3 inches for the example.


A diameter of 42.3 x pi 3.14159 = 132.89 inches. 1 mile is 5280 feet or 63,360 inches. Next divide 63360" by tire circumference 132.89" which gives 476.78 turns per mile.


Next enter desired speed say 65 then multiply 65 x 476.78 turns per mile and you get 30,990.7 turns per 65 miles.

Next divide the number of turns by 60 (minutes) which gives 516.51 turns/ revolutions per minute.



Then we take the final drive ratio and multiply it by the turns per minute at you desired speed and it will give you the engine rpm's in which ever gear you choose.

Example fourth gear on part one is 1:1 the rear diff is 4.75:1 giving a final drive ratio of 4.75.

516.51 turns x final drive ratio of fourth gear of 4.75 = 2453 engine rpm's at 65 mph.



Another example say the trans had a 5th gear with a ratio of .75:1 (overdrive)

.75 x 4.75 = final drive ratio of 3.56:1 . Final drive ratio x turns per minute = engine rpm.

3.56 x 516.51 = 1838 engine rpm.
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Old 06-08-2011, 11:58 AM   #4
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Re: Figuring out which differential gear ratio is right for

Another example let's change the diff ratio to 3.55:1. We will continue to use the at 545 fourth gear of 1:1 which gives a final drive ratio of 3.55:1

3.55 x 516.51 = 1833 engine rpm

If you want to know what engine rpm is at 70 mph with 3.55:1 you just take 476.78 turns per mile multiply that by 70 (33374.6) then divide by 60 minutes = 556.24 turns per minute.

3.55 x 556.24 = 1974 engine rpm's at 70 mph.

Let's see engine rpm at 70 mph if we had a 5th gear over drive of .75

.75 x 3.55 = 2.66 final drive ratio. 2.66 x 556.24 = 1479 engine rpm.
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Old 06-17-2011, 05:23 PM   #5
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Re: Figuring out which differential gear ratio is right for

Thanks for the info. Very informative.
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