[77newell]

Gear ration calculations were a mystery to me until I was looking at the Michelin tire website and noticed their brochures list the revolutions per mile for each tire.

Figuring out the needed drive axle gear ratio is now fairly easy. The variables in the calculation are tire size, axle gear ratio, transmission ratio, and engine speed. The tire size and transmission ratio are fixed. The question is what engine RPM do you desire when traveling at 60MPH (one mile per minute is used for easy calculation).

First look up the revolutions per mile of the drive tires. Michelin shows that my 11R22.5 load range G tires rotate 502 times per mile. My transmission output in 4th gear (highest) is 1:1 of the engine speed. If I wanted the engine RPM to be 2000 at 60MPH then the needed drive axle gear ratio would be 2000/502=3.98. If I wanted an 1800RPM engine at 60MPH the gear ratio would be 1800/502=3.59. If you have overdrive or under-drive through the transmission you have to factor that into the calculation.

If you are going to change a ratio you will need to check that both your speedometer and RPM gauge are accurate or at least know how far off they are so any inaccuracies can be compensated for. Currently I show 2025RPM at 60 (which I've timed to really be about 62MPH). Since my speedometer is reading low I need to calculate what the RPM would be at 60MPH by multiplying the 2025 by the ratio of the speed difference 60/62. The ratio of the speed difference is 60/62=0.968. The engine RPM at a real 60MPH is 2025X0.968=1960. To get the 502 tire revolutions per mile while the engine is turning 1960 requires a 1960/502=3.90 gear ratio.

Hopefully this helps those concerned about drive axle ratios. If you can spot any adjustments needed with these calculations please post your comments here so everyone can learn.

Jon Kabbe
77 Coach repowered from 555 to 6V92T
2007 civic toad