Fine discussion but no one has volunteered the formulas, here it goes.

ex: 215/50-16

215 = W = width (in mm's)

50 = H = height to width ratio (in this case 50% of 215mm)

16 = D = diameter of the rim (in inches)

R = ((W/25.4) x H) + D/2

R = ((215mm/25.4mm) X .5) + 8in = 12.23 in

take the width divide it by 25.4 mm's to get inches, multiply by the ratio

".5" and add half the diameter of the rim. This get you the radius, R.

Now do the following multiply R*2*pi(3.1416) and that gets you the circumference.

I hope this helps, I certainly used it in the past for stage1 and stage2 modifications.

By the way, bigger tire you're going faster than you think, smaller tire you're going slower than you think. I think...

rui

p.s. Please correct any typos, mis-information and other propaganda.