In the Datsun I am going from a 4.38 ratio to a 5.13. The main reason is so I can use only 3rd & 4th gear at the tracks I race.
The motor is making around 80whp with around the same amount of torque. The big increase in my mind is the fact that the motor will be 7-10hp higher up in the powerband.
Based on what I've found online it could be as much as a 17% increase but that seems rather high to me.
If anyone knows how to calculate a more precise number I'd love to know.
That is exactly correct - 5.13/4.38 = 17.12% increase in torque to the rear axle. (and 17% higher rpm for a given speed and gear)
In the end, you are looking for a way to compare "torque applied to the wheels" (or "longitudinal force applied to the contact patch") vs. "vehicle speed" for the two different final drive ratios?
Most of the lap time simulation stuff includes an input table for {engine RPM, power, torque}. Most of the simple "speed vs. RPM" calculators do not.
I recently came across a really old Excel-based calculator, "Excel Dyno V2.2": http://www.offroadvw.taylor-designs.com/exceldyno/index.htm
NMNA. Excel says the author is Wesley Taylor who started the file 8/30/2000 and last 'printed' it 4/26/2005. So far the macros have not taken over my computer, but I haven't actually used the thing yet. It's an impressive amount of work if nothing else.
If it does actually function, and you enter enough numbers, the 'Traction' tab might be useful (the dotted 'Ghost' lines are comparisons to a 2nd setup).
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