Transmission Guru Needed. 3 Gear Trans Question.

[JC3]

I'm looking for confirmation of my method of calculating my 3 gear transmission ratio and my final drive ratio.

Building my first 2.2 Sporty using Capra axles. Trying to pay close attention to gear ratios etc. Bought a compact three gear transmission with a ratio of 11.5:1. That ratio with the stock Capra axle ratio of 7.2:1 give me a final of ~80. I want to get down to ~50. I'd like to start doing that with pinion changes.

Transmission specs given to me (with 12T pinion) are: 48t locker gear to a 14t to a 40t spur.

With a 12T pinion, I calculated:

Gear ratio of 3.33:1 (40/12 = 3.33)
Transmission ratio of 3.43:1 (48/14 = 3.43)
Transmission final of 11.4:1 (3.33 x 3.43 = 11.4:1)

With a 20T pinion, I calculated:

Gear ratio of 2.00:1 (40/20 = 2)
Transmission ratio of 3.43:1 (48/14 = 3.43)
Transmission final of 6.86:1 (2.00 x 3.43 = 6.86)

Final Drive Ratio = (trans ratio) 6.86:1 x (axle ratio) 7.2:1 = 49.3

Still may end up with a smaller pinion due to space restrictions but I'm still closer than I was with at 12T pinion. Either way, just want to verify my thinking.

Any feedback appreciated.

[twade984]

Math seems to check out to me. Do you have enough adjustment to fit a 20t pinion?

[JC3]

Originally Posted by twade984

Math seems to check out to me. Do you have enough adjustment to fit a 20t pinion?

Thank you for the confirmation. I think that 20T would be a problem.

I have recalculated my final drive which I'm now basing on an overdriven front axle. The plan was to do that all along once I got my calculations confirmed.

With the overdriven front axle, I need a trans ratio of 10.6:1 and I can get there with a 13T pinion, so I'm in great shape now.

I end up with a final drive ratio of 52.8:1 which is exactly what I was after.