AndyA6

How do you calculate tire circumference? Don't know where to find formula, thanks!

xr4tic

circumference = 2 x radius x pi (or 6.28 x radius)

Ted K

C=pi R squared, with r being radius.

Ted K

don't forget, radius squared, or radius times radius.

=BootYMechaniC=

Nope, that's the formula for AREA of a circle....xr4tic is correct. =)

xr4tic

See, I paid attention in some of my classes

Au

<ul><li><a href="http://home.netcom.com/~sgalaba/tiresize.htm">Tire Calculator</a></li></ul>

AndyA6

THANKS!

RobertDinkel

Pi times diameter.

WildBlueYonder

Tire widths are measured in millimeters, so a 235/45ZR17 tire has a tread patch of 235 millimeters and an aspect ratio of 45. That means the sidewall is 0.45 times the width. So, to get the circumference of a tire...
pi x diameter =
pi x (wheel diameter + (2 x sidewall)) =
(using 25.4 mm/inch, wd = diameter of wheel in inches, tw = tread width in millimeters, pi = 3.14159, ar = aspect ratio as a fraction), then,

3.14159 x (wd + (2 x ((tw / 25.4) x ar)))

will give you the circumference in inches. Convert to other units as needed. This formula does not account for bulging of sidewalls, either. Best way to get diameter is to measure tire under load on the car. Best way to get circumference is to measure with a tape measure around tire. But this formula will help you calculate the right aspect ratio for a given tire width so that your speedometer still reads correctly.

Good luck.