Slightly smaller diameter tires a problem?

[Stevo]

Lookin' at putting 275/45/18's on all 4 corners. These are 27.7 in diameter compared to the stock 28.6 in...Anyone know if this will mess with the ABS, traction control, etc.?

I expect my speedo/odometer would be off by 3%.

[henri26900]

based on that tize when your speedo read 60MPH
your Speed is actually 58.1 Mph

This is a good website, sometime it takes a minute to load up.
http://www.miata.net/garage/tirecalc.html

[Sack Rat 2SS]

, You are correct that speedo, traction control, stabilitrack, and ABS will all be affected. However, you may be able to do a tune that will compensate for all of the abvove. Not sure what the purpose of changing sizes is about, looks or performance, etc. Tell us what your goal is, autocross roadrace??

[lawdogg149]

I want to go to the 18" setup for roadracing. Tires are impossible to get in the 20 inch. I have yet to find a 18" rim that will work without spacers.

[Stevo]

Tell us what your goal is, autocross roadrace??

I think with the 275's all the way around it'll grip better in the corners partly due to lowering the car by ~1/2 inch. Also with the smaller diameter's it should have more torque...

[Nine Ball]

Shorter tires will improve your effective gear ratio slightly, but not anything substantial. Also, a shorter tire will not only read a higher speedometer number, it will also put more miles on your odometer than you are really driving.