Originally Posted by

**MrShorty**
The thing that helped me get it straight in my head is to think in terms of torque/force rather than "effective gear ratio." The math looks like this (starting with an arbitrary input torque t0 at the driveshaft):

ta=t0*gr: ta is torque on axleshaft, gr is gear ratio

F=ta/r: r is tire radius (=1/2 diameter) F is Force between wheel and ground (this is what makes you go).

combined: F=t0*gr/r

solve for t0: t0=F*r/gr

for comparing when something changes:

t0=F1*r1/gr1=F2*r2/gr2 (remember that t0 doesn't change for our purposes)

if we assume 1 is the "stock" configuration, then to compare 2 to 1

F2/F1=(r1/gr1)/(r2/gr2)=(r1/gr1)*(gr2/r2)

Example: stock gear ratio 3.55 with a 30 inch diameter tire

new is 3.73 with 35 inch diameter tire

F2/F1=(15/3.55)*(3.73/17.5)=.90 so the new configuration will put 10% less torque (F2=90%*F1) to the ground and will thus feel more sluggish than stock.