Why is the 225/40R18 3.43% faster than my 225/45R18's but the 235/40R18's are only 2.2% faster?

 

225/40/18 has a diameter of 25.1" and 235/40/18 has a diameter of 25.4", which is closer to the 225/45/18 diameter of 26.0".

 

The diameter of the tires is based off all 3 of the #'s 225 is the width in mm (metric) then 40 is the aspect ratio for the sidewall so 40% of 225 mm is the side wall of the tire. you double this then add the rim size which is in imperial 18" (you have to do some converting ) to get the diameter. Then figure out the circumfrence pi x d or just use the tire size calculator
http://www.miata.net/garage/tirecalc.html

 

Oh alright, thanks guys.

 

That's a very common mistake . Most people think that if it's the same sidewall series, it will be the same height.

 

235/40's are the best tire size..!

from 215/45R18 to :

235/40's 0.8% too fast / 60MPH = 59.5MPH
225/45's 1.4% too slow / 60MPH = 60.8MPH
225/40's 2.1% too fast / 60MPH = 58.8MPH
215/40's 3.3% too fast / 60MPH = 58.0MPH

the best fit sized tire for the cobalt are 235/40R18... nothing else..!

 

For the SS/SC maybe.......the regular SS has 205/50R17 which is shorter than the 215/45/18.

 

well for a SS the 225/40R18 would have only a 0.1% difference.. i dont understand what your question is...

but he has a SS SC.. so 235/40R18 are the best spec sized tire.

 

I meant that your info is correct, but you should really specify the model instead of just saying Cobalt. Many people seem to be confused on tire size issues.

 

ok.. i understand now.. but the guy who started the thread has a SS SC... so.. thats why i said that.. if he would have something else i would of known