does the total diameter of 18's with 225/40r18's equal the total diameter of 16's with 205/55r16's. if not which has a bigger total diameter? thanks.

The 225/40R18's have a slightly larger diameter; 25.1in vs 24.9in. That's pretty darn close and about as good as you're gonna get with standard tire sizes.

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

Originally posted by Mannyyy
The 225/40R18's have a slightly larger diameter; 25.1in vs 24.9in. That's pretty darn close and about as good as you're gonna get with standard tire sizes. http://www.miata.net/garage/tirecalc.html

thanks a lot.

Here's a formula for figuring overall rolling dia.

Tire width(225)x Profile (as a decimal .45)x 2 divided by 25.4 (always) + rim dia (17)= rolling dia. (24.97 in this case)

It will give you exactly what you need and you can use this to compare potential wheel sizes for speedo cal and such...

This formula is also in this forum posted by another fine 2gnt member.
Credit goes to him.
:thumbsup

Or go to Tirerack.com. they can calculate what the +size tire for you is.

They say 225/40-18

thanks a lot for the formula. works great. as for tirerack, i ve tried that too but i just wanted to know the total diameter to see what i m up against when i finally get the rims. also, so i can check if i can go with aq bigger tire to fill that gap. thanks again everyone.

generic formula for +1 sizing is +10/-5 +1. basically add 10 to cross section and subtract 5 from profile for every +1 in diameter.

A.E.,
Didn't mean to steal your thunder on the formula...Thanks for posting it! Has helped lots recently! Also for the plus sizing info!