hey guys(and gals ), i'm an avid wrencher but not much of a suspension guru. so my question is if i get an alignment now with stock wheel/tire combo, then swap on 18x8.5 w/235/40/18's, do i need another alignment and vice versa? the dealer told me yes, but i wasn't convinced. that would mean if you run a different size tire/wheel combo at the track your alignment would be off, correct? any help with this question would be appreciated and thanks for your time.

 

c'mon, someones gotta know this! suspension guru's, where you at?

 

If the diameter of the new tire is close to the previous set, you don't need an alignment. And, yes, an alignment will only be correct for a small range of tire diameters, so if your street and track tires are dramatically different, your alignment will probably be a compromise.

 

going from 235/45/17 to 235/40/18. they're pretty close, so i should be OK then?

 

Yeah, no problem. According to http://www.miata.net/garage/tirecalc.html, the diameter is a tenth of an inch different:

 

cool. thanks for the link to that calculator too, very helpful.