Ok, I’m sure this is simple for most. But I’ll admit I’m still learning. Anyways, here’s the question:
how do you know if one set of wheels has the same diameter as another?
eg. - 45/17R and a 40/18R. Are they the same diameter?
or
eg. - a 35/18R and a 40 17R.
etc etc etc…
I’m sure there’s any easy way to calculate this. I’m all ears… 
Leo.
Ok, first you need to know what those numbers mean.
Take 255/40R17:
-255 is the width of the tire in milimeters
-40 is the height of the sidewall measured by the percentage of the width (so 40% of 255)
-17 is the rim size in inches
So you do this…
255 * .4 / 25.4 * 2 + 17 = overall tire diameter
255 * .4 = 102 This is the height of the sidewall in milimeters.
102 / 25.4 = 4.0157 This is the height in inches.
1.0157 * 2 = 8.0315 Because there is the bottom sideall and then the top sidewall.
8.0315 + 17 = 25.0315 This is the overall diameter of the rim.
205/60R15 (S13) is 24.68
205/55R16 (S14) is 24.88
As long as you are within 5% of the original size, your speedo will be fine.
What can I say?
Thanks for all the input! Both posts were very usefull…
Leo.
This isn’t tire related but has lots of really good technical info regarding wheels if anyone needs it.