gsrblack

I'm trying to figure out how a higher or lower revs per mile would affect the reading on our speedometer and odometer.

I looked at the http://www.miata.net/garage/tirecalc.html site and am a little confused. Is this correct thinking?

If the stock tire is spec'd at 819 revs per mile, what would a higher or lower rev per mile mean?

Is this correct? If the new tire has HIGHER revs per mile (smaller diameter than stock), I'm assuming that the speedometer on our car would show up faster than what you actually are travelling at. That would mean that the odometer would show up with more miles than what you actually travelled. And your calculated mpg efficiency based on the odometer reading would be better than what you actually drove.

If the new tire has a LOWER revs per mile (larger diameter than stock), than the speedometer would show a speed slower than what you are actually travelling at and the odometer would show less miles than what's actually travelled. The calculated efficiency would be worse than what you actually did get.

Is that right thinking?

Do you think that a tire with about 10-15 revs more (or less) would yield a significant amount of mileage difference over, say, 30,000 miles?

ScottinSoCal

15 more revs per mile would add up to a difference of 549 miles every 30K. So your car would have traveled 30K, but the odo would read 30,549 more. While traveling at 70 mph, your speedometer would report 71.3 mph. The difference is less than 2% (1.83%), and I'd guess that's within acceptable tolerance for the speedo, anyway.