Tire size vs gear ratio math

awing

New Member
Alright, I was hoping someone could tell me if I was wrong or right here. (I chose to put this in the race section, as this applies to quarter mile times)

I have the stock 265/75 R16 tires provided on the nismo trim. I am going to the 18" nismo wheel, and Nitto has a wheel thats 285/50 R18. I went over to http://www.miata.net/garage/tirecalc.html and plugged in the numbers.

The new tire will have to do 7.7% more rotation for the same speed as the old tire.

If I apply a 7.7% increase to my 3.69 gear ratio, I get 3.97.

So is this the same as running a 3.97 rear with the stock tires?
 

mdawg4x4

Administrator
How about: New ratio should = Current gear ratio/old rolling radius*new rolling radius

The reason I use rolling radius (distance from center of tire to ground) is different tires have different sidewall structure letting the sidewall flex a little different under the weight of a vehicle.

A good approximation would be: New ratio = current gear ratio/old tire diameter*new tire diameter
 

Shift_Avenger

Active Member
awing said:
Alright, I was hoping someone could tell me if I was wrong or right here. (I chose to put this in the race section, as this applies to quarter mile times)

I have the stock 265/75 R16 tires provided on the nismo trim. I am going to the 18" nismo wheel, and Nitto has a wheel thats 285/50 R18. I went over to http://www.miata.net/garage/tirecalc.html and plugged in the numbers.

The new tire will have to do 7.7% more rotation for the same speed as the old tire.

If I apply a 7.7% increase to my 3.69 gear ratio, I get 3.97.

So is this the same as running a 3.97 rear with the stock tires?
That is a good approximation. Obviously the whole purpose would be to not have to shift to 4th before the end of the run which happens to me every time.

Are you Auto or MT?
 
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