Can someone who knows for sure tell me the factory spec for tire diameter so the speedo will be 100% accurate? I'm going to get new wheels/tires, and I want to make sure I don't throw off my speedo/odometer. I have heard some people write that the factory tires are not the correct diameter. So, there has to be a spec written somewhere on this matter. Thanks, -JDB.

 

Here you go....clicky...it's self-explanatory

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

 

Just off the top of my head, the numbers I've seen here at ScionLife for xB's running on factory rubber, 185/60R15, the speedo is about 3 to 4 percent optimistic (it indicates 3-4% faster than your actual speed).

If this is true, then the stock factory tire on the JDM bB, which is a 185/65R15, I believe, might be the correct size, since it rolls about 3-4% farther with each revolution.

This is all, of course, theoretical unless you validate the initial accuracy of YOUR speedometer and use that as the basis to start from...

 

I heard that too, but I wonder if there's any truth to it

And besides, the 185/65/15 tires are much cheaper than the stock xB 185/60/15 tires

 

the tire calc backs up your statement tomas, if the bB tire is 185/65 then the xB 185/60 tire will cause the spedo to read 3% faster, 60mph on the speedo = 58.2 on the radar gun

 

moved...