Quote:
Originally Posted by XB101
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Running 245/40R18 tires instead of the OEM 245/50R18's will, in fact, result in an error of 7.7%
SIZE *****Revs./mile **Speedo and Odo Error
245/50-18 **755 ***** none
245/45-18 **784 ***** +3.8%
245/40-18 **813 ***** +7.7%
Another consideration is the tire load rating which for X3 OEM tires is usually between 98V and 101V. Some 245/40-18's can have load ratings as low as 96V, which would be safe, but would require higher inflation pressure and between the low sidewall height and high pressure would probably result in a not very comfortable ride.
P.S. That Miata Tire calculator has substantial errors. It does not report the correct rev./mile numbers for some sizes. It's best to check the manufacturer's data sheet and then do your own calculations.