According to my calculations, the 215/60-16 has a circumference of 82.18 inches, and the 235/45-17 has one of 79.57, which is about 3.2% smaller... so I wonder which size has the more accurate speedo reading! Anyone know?

I don't think there's any 'setting' to tell the car which size factory wheels/tires it has, so I guess one of the sizes has the speedo off by 3.2% or they split the diff, one is 1.6 high and the other is 1.6 low. Even if one is 3.2 off, then that's still only 2mph at 60, no biggie.

 

Bit, you are absolutely right on your calculations, I can't believe I understood all of that. My mind was crunching away when reading your post...I think I read it too fast

IF the GS is set up to be more accurate with the 16in. tire/wheel combo, then making the GS tires 245-45-17 instead of 235's would make the ratio of the two closer together.

Check out the diagram below from http://www.miata.net/garage/tirecalc.html

The grey and black is the 16in. and the outline is the 17in. tire/wheel. With the standard 16's vs. the standard 17's the rev per mile is 785.2 and 796.3 respectively.

When you change to the 245-45's on the 17in. tire you have 785.3 revs/mile. Much closer to the stock 16's.

I've done calculations like this by hand a few months ago and this web-page makes it much easier. It's pretty accurate too. Of course none of this lets us know which of the GS tire/wheel combo is closer to accurate for the speedo, so changing the tire size on the 17's may not help.

 

Usually each large city has a speedometer shop or two where you can take your car in for an accuracy check. They put you rear wheels on a roller similar to a dyno (not tied down as the rollers have no pressure applied to them). They then compare your speedometer reading to theirs which has been calibrated to the rollers. The one I have used then gives you a card to keep with your readings versus actual.