i know people have been using yellow boxes for different gearing but is it recomended to get this also with different profile tires?? if so and you didnt get this box what would be the down effects more miles on the car and wrong speedometer reading any other peoples inputs???

 

venture to guess unless you are using radically different profile tires the difference is pretty minimal.

tire calculator you can run some numbers on:
http://www.miata.net/garage/tirecalc.html

 

Look at TireRack and see the difference in your tire diameters. Usually not enough to matter much.

 

switching from Goodyear F1 Asym 245s to Summitomo 255s actually got me more accurate if you trust my TomTom. It was a matter of 2 mph low with the Goodyears at 75, so not a big deal either way.