Okay so my speedometer read 138 on I-95 N the other day driving from Vegas to Reno. I have 19"s though so I am guessing there is about a 2-3% error which would bring it to about 142 or so. I kept it there for about 10-15 seconds before scaring the bejesus out of myself on a turn. Thought I'd pass that along for all you speed junkies.

 

That raises an interesting (to me, anyway) question about the effect of larger-circumference tires on the US-spec 130 mph speed limiter. If the larger tires cover more distance for each rotation, and the speedo (and thus the speed limiter) measure speed by wheel RPM, then larger-circumference tires would "fool" the speed limiter to the extent of the percentage increase in circumference. Even if the speed limiter measures speed indirectly (via engine RPM and transmission gear) it would still be "fooled" by a larger tire circumference, no? Okay.

 

Correcto! Here is a great site that allows to calculate exactly how much the diff is:

http://www.miata.net/garage/tirecalc.html