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