Miata Tire Size Question

I'm getting ready to order some tires for my 96 Miata.

205/50-15 seems to be the natural choice for track use, and I already have a set ready to go.

For the street, 195/50-15 would be a direct Plus 1 swap up from the stock size of 185/60-15.

However, with another set of 205/50-15's, my alignment wouldn't change when I swap from street to track tires, but the speedometer and final drive ratio would be slightly off, due to the 205's greater circumference.

Is there enough of a difference to matter?

Woody wrote: Is there enough of a difference to matter?

This will tell you exactly how much difference.

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

Personally, I like 205's on the street. The taller side wall takes away some of the harsh ride if a stiff suspension.

It would depend on the specific tires, but my rough calculation puts the circumferential difference between a 195/50/15 and a 205/50/15 at something like 1.7%. That's not going to create noticeable speedo error.

jrw1621 wrote:

Woody wrote: Is there enough of a difference to matter?

This will tell you exactly how much difference. http://www.miata.net/garage/tirecalc.html

Wow, I never saw that before. Thanks.

I think that my Miata's speedo is about 4% optoimistic anyways, so it might not be a bad thing.

Unless I'm mistaken, aren't there subtle differences in tire diameter anyway? Obviously there must be, since tires will get smaller as they wear down.

I was under the impression that even though a 205/50/15 is theoretically bigger than a 195/50/15, in practice there's going to be more variation between tire models within a given size than there will be between sizes.

Zacktly. That's why, while the tire calculator is cool, I always work from manufacturer spec sheets.

mistanfo's dead on about the Miata speedo as well. They're very optimistic, so running a taller tire is only going to make it more accurate.

That settles it. 205/50-15 for road and track.

won't matter.

i run 195/55/14s as my snow tires (a bit taller than OE), and don't notice any difference from my 195/50/15 summer/autoX tires, except for a slightly cushier ride.