Tire sizes

Over in the SEC forum, a build thread got hijacked by tire talk and what is the best tire. Consensus is the Cooper STT.

I need a 31" for a 16 inch OEM rim. I looked up 31 x 10.5 x 16 on the Cooper STT but found none. The STT comes in the following sizes for a 16 inch rim. Are any of these equivilent to the 31? Sorry, but I don't really understand how tires are sized.

225/70R16
225/75R16
235/85R16
245/75R16
265/75R16
285/75R16
305/70R16
315/80R16
325/80R16

the formula for converting metric sizes is as follows.

Multiply the treadwith by the aspect ratio, 245 x .75 = 183.75

Then multiply that by 2 which = 367.50

Then divide that by 25.4 which = 14.47

Then add the rim diameter of 16 which = 30.47

30.47" is the overall diameter of a P245/75R16 tire, so a 30.5".

A 265/75R16 is a 31.65".

To find out the width in inches, multiply the 245 by 0.03937 which gives you
9.65" in width. So it works out to a 30.5"x9.5" tire roughly.

The 265 is 10.4" inches wide.

I have a 2001 with P245/75R16 Goodyear Duratracs on the factory Icon wheels and love 'em.

i don't know, but i think i know which thread got hijacked, lol.
STT's FTW!

If you are too lazy to calculate it yourself, like I am, this should help a lot:
http://www.miata.net/garage/tirecalc.html

You can just keep tweaking the numbers till you find something that is about right. I usually open the size listing for the tire I want, open the tire calc on the other screen, and go down the list from cheapest to most expensive sizes for the rims I have till I get a combo I like. Only thing it doesn't seem to calculate is tread width, which you can calculate easily by dividing by 25.4 (tread width is listed in mm) or multiplying by .03937 like Ian said.

The width in the metric sizing is the "nominal width", or the widest part of the casing, which is usually not the tread area. In the same size of tire, tread width can vary according to brand or style. As an example, my 245 (~9-5/8") has a tread width of only 200mm (~7-3/4"). The physical dimensions of the tires themselves will also vary according to brand and type. My 245/75/16 is just a bit under 30" in diameter- with full tread, it would be half an inch under the calculated diameter, which would also change the circumference by something like an inch and a half.

I normally refer to Discount Tire's calculator- as it converts Metric sizes to standard dimensions and will tell you indicated speed differences, revolutions per mile, etc.
edit- I guess the Miata.net page does this as well, except for the speedo differences.

http://www.discounttire.com/dtcs/infoTireMath.dos