Syn

I'm debating buying a set of stock C5 rims which have 245/45/17 tires up front and 275/40/18 tires in the back. When I plug these sizes into the tire size calculator at http://www.miata.net/garage/tirecalc.html I see that the rears would show as 4% too slow.

So my question is: Does the speedometer use the front or rear wheels for its reading? Or both somehow?

Also why would these tires be so much different in diameter? 275/35/18 in the rear would be much closer to the fronts than 275/40/18. It seems odd that GM would use such a differing combo.

Mike in LA

Someone correct me if I'm wrong but I think the speed sensor is on the tailhousing of the transmission. I'm not quite sure what you are asking in the second part of your question. Are you trying to figure why there is the 4% between the sidewalls of 35 and 40?

Syn

If the speed sensor is on the trans then it would stand to reason that the diameter of the rear wheels is what matters when calculating the accuracy since they are the drive wheels. I just wasn't sure if the sensor was elsewhere.

The second part was simply asking why GM would use a tire on the rear that is so much larger than the front on the C5. 245/45/17 and 275/40/18 are not the same diameter. For an 18" rim, 275/35 tires would be much, much closer in size to the front 245/45/17 (which is also nearly identical to the stock 245/50/16 tire size on the F-body). But GM went with 245/40 instead. It just seemed odd to me.