I have 20s so is my odometer really going that fast or slower/faster than it really is?

 

Use this link to find the answer to your question and what size tire you should run to keep the odometer as close as possible:
http://www.miata.net/garage/tirecalc.html

 

It really depends on whether you keep rolling diameter as close as possible to stock. Read up on the plus principle.

You can always get your speedo calibrated.

 

If you used tires that maintain the same circumference as stock, your odometer will read the same as before. I'm not exactly sure how an odometer works, but I'm guessing that it counts the number of rotations that the axle makes and then converts that into miles using the circumference of the stock wheel/tire setup. If that's the case, if you used tires with a greater circumference than stock, one full rotation of the axle will correspond to MORE distance than stock because of the larger circumference. This means that you will travel farther than your odometer is expecting, so your odometer will read LESS distance than you actually traveled. LARGER circumference than stock => odometer reads FEWER miles. Similarly, a smaller-than-stock circumference means that your odometer will read MORE miles than you actually travel.

^^^ I don't know how an odometer actually works, so this is just a guess.

I don't know what size tires you have. Chances are, the circumference isn't smaller than stock, so your odometer is probably reading either the same as before or a little less than it should be.

 

All right, what size tires are you running, then we can do the calcs and let you know. Or you could look it up on the link above in Post 2