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):Quote:
I'm starting to confuse myself thinking about it more and more trying to make sense of it and how to explain it.
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).
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
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.