Tuner

[RazorbackMustang]

Unless you're putting some ludicrously-wrong-sized tires on, your speedo won't be off by that much.

For those of us with some degree of OCD regarding those things, it would be enough to annoy me. Hell, I changed the revs per mile on mine when I borrowed a friend's drag tires....so my odomter wouldn't be off.

 

[Sky Render]

For those of us with some degree of OCD regarding those things, it would be enough to annoy me. Hell, I changed the revs per mile on mine when I borrowed a friend's drag tires....so my odomter wouldn't be off.

LOL! I'm not resetting my revs per mile twice a week when I put my racing tires on and then take them off!

 

[tagle281]

Well hell Nitto used to put the revs per mile on their site.

 

[RazorbackMustang]

LOL! I'm not resetting my revs per mile twice a week when I put my racing tires on and then take them off!

Yes, I'm that retentive.

 

[jaymoo7]

where did you get that number?

I was under the impression that not all 20" tires have the same Revs Per Mile.

I talked to Chandler at Brenspeed about it a while back for mine and the Sct is in increments of 5 only. 705 was the closest to the actual number and worked like a charm. Used my how to make sure and it was dead on. They will all give you a number and most of them will be closer to 705 for 20".

 

[stkjock]

wheel size DOES NOT MATTER

It's the tire size that matters

 

[frank s]

I use math. Tire diameter x 3.142 = circumference. 63360 inches in a mile so divide 63360 by the tire circumference and you have revs per mile for your car.

Just for some Data Points, here is the Tire Rack specification comparison among the tires I had on hand back in 2009 or so:
http://www.fototime.com/CA4DDFBF71AE628/xlarge.jpg

Seems to me it would be easy to make a measurement error, and the circumferences are pretty close together. Also close together are the actual Miles Per Hour at indicated 60 MPH. I'd say that unless you are changing tire sizes radically, this is an academic exercise, usually falling within the eyeball vs. speedo-needle error expectation.

(I won't bring up the Manufacturers' Tire Diameter vs. Effective Radius issue. )