How do I get the right tire size to stay as close to stock for the total diameter of the wheel (rim + tire)...

For 18x7.5 and 18x8.5

I calculated 215/35/18 and 245/30/18 which is roughly a half inch taller in diameter... however I am having a hard time finding someone who sells the 245/30/18 tire size... is it because the sidewall is too thin?

Do the rest of you guys just buy tires w/ higher sidewalls? How does that effect our speedometer?

 

You can do a search, but I believe clostest to stock diameter would be

front - 225/40/18
rear - 255/35/18