17" to 18" wheel
 

Found a great set of 18" FX wheels with goodyear wranglers attached (70% tread left) for $350.00. Will they screw up my speedo? Is the computer programed for the stock 17's?
The guy that has them had 17" originally and had these and he said speedo wasn't affected.

It should be fine might mess with it a small amount but not enough for you to notice.

the only time the speedo will be affected is when the diameter of the tire (not the wheel) is affected. a 31" tire on a 17" rim is the same as a 31" tire on a 18" rim. but if the tire on the 17" is, lets say, a 31" tire and the tire on the 18" rim is a 32" tire, your speedometer will be slightly slow, but nothing major. If you were going from a 31 to a 33 or 35, then you might run into your speedo being noticeably slower.

to figure out the change, check this page out: http://www.miata.net/garage/tirecalc.html

I think I just talked myself out of the sale as I'd have to buy a 18" wheel and tire combo for the spare.
It was a good deal tho, hard to pass up.

according to? the wheel size does not matter... its the tire size. if the 18" wheels have tires that measure the same diameter (or within a half an inch) as the tires on your 17", you'd have no issues.

some people crack me up!!! lol

You can't fix stupid