What tire diameter are rear gears based on?
 

Someone w/a truck with, say, 30" diameter stock tires has 3.42 gears this can't be the same EFFECTIVE ratio as someone with an, oh, let's say a Monte Carlo with 28" diameter tires with the same 3.42 gears. A gearhead buddy of mine says that ALL rear gears are 'based' on, or rated for, a 26" diameter tire. Anyone care to confirm or deny?

WTF??

A gear is a gear. It doesn't matter what size tire you are running it's still the same gear.

IE a 3.42 gear on a Camaro with a 26" tall tire is the same gear as on your truck with a 33" tall tire..

Now, your overall final gear ratio will differ GREATLY in the case above. If 2 cars have the same rear end gear but have different size tires, the one with the shorter tire will have the lower effective gear ratio (numerically higher) and usually be the one who launches harder if all things are equal.

I know that a gear is a gear - I guess my quandry is that when two separate vehicles have a 3.42 gear set, that means physically that the ring and pinion are identical, but the effective gearing is totally dependent upon the relation between the tire diameters on both cars. In other words, 3.42s on my truck with 31" tires have a lower effective ratio than the exact same 3.42s on a different vehicle with 27" tires. Same gears, but different final drive ratio (all else being the same).

Exactly what I'm trying to figure out. If you get smaller diameter tire your gearing would act as if you had a lower ratio axle.

3:73s feel like 4:10 for example if you go from a 265 to a ????

Hey Guys...

Final drive ratio (like you'd find listed in Motor Trend or similar car mag) doesn't compensate for rear tire diameter. Final drive ratio is the transmission ratio (lets say .70:1) times the rear end gear ratio (lets say 3.73:1). Final drive ratio for the example above is 2.611:1.

There are a number of gear ratio / mph type calculators online (I'd suggest searching around a bit). Most of them require you to have tire diameter (won't let you punch in 275 / 40 R17, you have to know that tire has an od of approx 25.7").

And you will need all the numbers in the size designation to figure it out. 265 by itself won't help you (thats the section width). You still need the aspect ratio (40, 50, 60, whatever) and the rim diameter (15, 16, 16.5, 17, 18, etc).

It is fairly simple math to determine what size tire will make 3.73's feel like 4.10's. Basically you need a tire that is 91% of your current tire diameter or 9% shorter (found that by dividing 3.73 by 4.10 = .9097 (91%)). If you've got a 28" tall tire now, you'd need a 25.5" tall tire (28" x .9097 = 25.5") to have it "feel" like 4.10's. If you've got a 30" tall tire, you'd need a 27.3" tall tire. At that point, you've got to find a tire that would give you the size you want (I'd suggest looking at tirerack.com's website, there is alot of information there).

'Dreamin'

A 10% change in gear is the same as a 10% change in tire height-


3.73 4.10
---- = ----
26" (x)"

X= 28.579 or roughly 26 X 1.1
Most GM gears are 10% apart 2.73- 3.08- 3.42- 3.73- 4.10- 4.56 are all pretty close to 10% apart- with the obvious exception being 3.23. Other gears like 3.90 and 4.30 are aftermarket ratios since the early 70's.

This may help.

http://gs.tolan-hoechst.com/tirecalc.htm

Thats a fairly useful calculator. And I'm sure there are a bunch of other ones out there. Personally, I've got an Excel spreadsheet that I made up to compare transmissions (60E vs 80E), rear gear choices, and tire diameters vs engine rpm at cruise speed and shift points.

'Dreamin'

I would need some tiny tires for my 3.23s not too suck.

Go figure I had to choose a word that had an actual meaning other than the way I used it. LOL