Do CAD all day. Cant think. How much would one inch additional tire diameter affect gear ratio? Better yet, a formula to figure it out. Going from 18's to 20's.

 

You could use this http://www.supermotors.org/vehicles...ators/index.php
But if you wanted to calculate it yourself you would have to first find the circumference of the tire.
C = pi*diameter

since you want your final answer to be in mph, you could convert that into miles
Circumference*(1 ft/12")*(1 mile/5280 ft) = miles/tire revolution

So to find what speed you would be doing at a given rpm you would have to do
Engine RPM * (transmission output/transmission input) * (rear output/rear input) *(tire circumference/rear output)*(60min/1 hour) = speed in mph
Sorry if that got kind of confusing

 

Thats was good..... But I need to find out how it will affect rear gear ratio.
Example: 26 inch tires with a 4:10 ratio. New tires 28" I know its still a 4:10 no matter what. I really wanted 3:90's based on 26" tires, but I knew I would go to a larger diameter tire. Sorry if that got kind of confusing. Just wondering if I was closer to my goal.
26" circumference = 81.6814.
28" circumference = 87.9646.
So I can travel 6.2832 inches more per rev with the larger (20" rims) tire (at the same ratio).
Looks like I just need to multiply the circumference by the ratio both ways and just compare the distance per rev. Does that sound right?

 

HP Tuners has a way to do it. if you give me the exact tire size change(either in inches or 275/40/17 form) and gear ratio, i should be able to do it. its used to adjust the speedometer, but i should be able to figure out for you

 

Its just a ratio. 26/28 * 4.10's = 3.81

going from a 26" tall tire to a 28" tall tires lowers the axle ratio to 3.81.

28/26 * 4.10's = 4.42

Thats the axle ratio you need to counteract the taller tires.

 

Thanks. That is good news to me. I felt that 4:10's were a bit much but I knew I would have to compensate for the larger diameter. The 3:81 sounds a lot more where I wanted to be with a built A4 and a Yank SS3600.

 

Don't make this difficult. It's not.

You could just multiply the inverse ratio of the tire diameters times the gear ratio to get the new "effective" ratio.

eg. 26/28 x 4.10 = 3.81

Because circumference is directly proportional to diameter (via PI), all that stuff drops out when you do the math.

Of course, you could go to TireRack website, check the specs for the actual tires you are going to use, and compare the revs per mile using the same formula.