I re-measured my old wheels and tires. I thought they were 25 inches in diameter. Turns out they are actually 26.5 inches. Does anyone know what the diameter of a stock, mounted and inflated tire is? With my old tires, my speedometer was spot on at 70 mph and I'd like to keep it as close to that as I can when I buy new tires.

Steel wheels, 1970, Michelin 165SR15 measured 25 inches, one on the car, spare on the ground same.

Dang pilots! Next thing you know you will be putting a timer on the dash and making "standard" rate turns when its foggy. I have an older Panorama article out and it list the "Theoreticle Unloaded Diameter" of a the original 165R 15 tire as 25.7" Yes I am splitting hairs too. 205/55/16 vs 205/50/16...

Love the new cell. March '95

You can calculate the diameter with this formula:

Dia = [Rim size in inches] + 2 x [(Width in mm)/25.4] x [profile]/100

Ex: 165/80/15

Dia = 15 + 2 x 165/25.4 x 80/100 = 25.4 inches

Or go to this site:

Tire Size Calculator

This is the tire calculator that I prefer.

http://www.miata.net/garage/tirecalc.html

I found that a 205/65 15 will barely fit width wise but is almost perfect for diameter. Kept my cruising speed the same and looks nice under the fenders. I have pedrini's and the fit is perfect but tight.

or this and look where I found it http://www.914world.com/specs/tirecalc.php