Effective Gear Ratio

[MrShorty]

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

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.

[the phantom]

Also some things the math does not take into consideration is if your lifting your truck you will have more wind resistance and also if your adding any extra weight from larger tires and rims. It might not be much but it is a factor. So my recomendation is to always round up to the next available gears or even the next after that. Just my .02.

[Crawdaddy]

Moved to the Lifted and Offroad Suspensions sub-forum and stuck.

[cbroberts88]

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.

At first this hurt my brain but once I broke everything down it became very simple. Great formula!

Moved to the Lifted and Offroad Suspensions sub-forum and stuck.

Thanks!

[K15 Blazer Guy]

wait, im confused... so...

(33 / 29) x 3.55 = 4.03

to run 33" tires, i should have at lease 3.83 gears for the axles?

i have 265/75 (about 29") and i want to go to 285/75 (about 33") .... i still dont know what gears i should have...

[dsfloyd]

wait, im confused... so...

(33 / 29) x 3.55 = 4.03

to run 33" tires, i should have at lease 3.83 gears for the axles?

i have 265/75 (about 29") and i want to go to 285/75 (about 33") .... i still dont know what gears i should have...

so with the formula you used at a pure conversion to get near a stock gear ratio you would need 4.03 gears. I dont believe they make those so after that it is based on your needs. you have a 4.10 or a 3.73 I believe as options. A 4.10 is going to get you close to the stock ratio but a tiny bit shorter ratio which will give you a little more ability to get the blazer moving.

look at it this way as well
(29/33) x 4.10 = 3.608 effective gear ratio so real close to 3.55
(29/33) x 3.73 = 3.28 effective gear ratio so taller than stock by a decent margin and a little more effort required to get things rolling.

[K15 Blazer Guy]

ooooo-k cool thanks! =)

im gonna stick with my 265/75R16s

31" looks, and handles well with my 3.73 axle

[K15 Blazer Guy]

although, 32 would almost be 3.55.... just a little under!