I need to know which tire size would work better without throwing the speed sensor off much, a 215/40-17 or a 215/45-17 size tire, also would the 215/40's be thick nough to avoid bending the wheel on the first pothole you run since I'll probably get the yoko parada spec2's.

 

did some more research, the 215/45-17 keeps the speed sensor closer to spec and it's also better for ride comfort and to avoid wheel damage.

 

...that's (215/45/17) what I'm running on my Sub-zeroes. Rides like stock with my Eibach prokit

 

I have the 215/45/16 Azenis tires which still thows off the speedo (speedo says you are going 70 but you are really going 65)......with the 215/40/16 you can feel every bump on the road........

 

is it the width of the tire or the side profile that throws off the speedo or both
is there anyway to fix soemthin like that

 

There is a calculator on the net that shows you how much your speedo will be off plus or minus if you use a different tire size http://www.miata.net/garage/tirecalc.html there isn't a way to fix it...........

 

Its the diameter that changes the speedo reading.

This can be potentially confusing for some of us here, since the stock tire diameter for the RSX and the Si is different.

The same 215/45/16s on my EP3 only throw the speed off by 2.5%... meaning when MY speedo says 70, Im going 68.25.