GT40s Supporter
GT40s Supporter
I'm always interested in another calculator

I like to plan ahead. I'm tired of swapping parts till I get what I want.


Howard Jones

GT40s Supporter
Here's a gear ratio, rpm. speed calculator. Plug in the ratios and tire size you want to use and it will give you RPMs and speed in each gear. Now what you need is available ratios for the gearbox you are interested in.

Google is your friend. I found this one in less than 2 mins.

As a primer here's what you need to decide:

tire size, target cruse speed in top gear, maximum useful speed in each gear, useful rpm range of engine, top speed in highest gear.

Here's my recommendations (aprox): tire diameter=26"
75mph cruse,
1st=60mph, 2nd=85mph, 3rd=120mph, 4th=145mph, 5th=170mph
engine rpm= max 6500 best range tq 2500-5800
max speed 175-180

Most 5 speeds will need an aprox 3.50 to 1 final drive ratio, 2.6 to1 1st gear, and a .8 to1 5th with the others spaced in between. Some thing like 1=2.6 2nd=1.7 3rd=1.3 4th=1 5th=.8 with speeds of 55,84,110,142,178 @ 6000rpms

The above will get you a nice road ratio box that is plenty fast enough for track use. With 600hp I think that most tracks will only need 3rd,4th, and 5th. 600hp will really limit the number of transaxles available. plan on spending somewhere in the range of $15000-$20000 for the power train. Ask your gearbox vendor about required maintenance intervals at that power rating. Especially if you run slicks on track a lot.
This is the one I used for figure out which drop Gears I wanted to go with. It’s pretty good.
That is pretty good. I checked the math, and it is sound, on the example I tried there was less that a .2 MPH difference between my math and theirs, and that is probably due to how our calculators round. Made be rethink my tire size though. as clearly over 200 MPH is better that barely over 200 MPH. The wind resistance and power might make 200 MPH possible, but I would not press my luck, and don't trust the aerodynamics enough.

The way that calculator laid out a chart in 100 RPM increments, was very nice, I was not planning on being quite that thorough, nice touch. Being aware of this now, I won't bother. Thanks!