Can I have a hand with my regearing math?

[vanguard]

My plan is to move from tires with an actual diameter of 30.6 to 33.3. I am also going to regear from 4.1 to 4.56. My question is, what impact will this have on my speedometer?

I *think*, it will read 2.2% faster but I'm not sure I'm doing this right. Care to give it a shot? Please show your work.

[pskhaat]

30.6 to 33.3. I am also going to regear from 4.1 to 4.56. My question is, what impact will this have on my speedometer?

I *think*, it will read 2.2% faster but I'm not sure I'm doing this right. Care to give it a shot? Please show your work.

Rotation is directly proportional to radius and thus also diameter and is roughly proportional to vehicle velocity, thus:

33.3/30.6 ~= 1.0882
1.0882 * 4.1/4.56 ~= 0.9785
1 - 0.9785 = 0.0215

or

(new_tire*old_gear)/(old_tire*new_gear) = speed_diff

So yeah, you're correct in that you will read ~2.15% faster.

[vanguard]

Thank you