Without spending $300, to take it to a specialist, can I calibrate my wifes speedometer myself? On my jeep I can just swap a gear inside the transfer case. Anything like that on these? I'm thinking of putting some new tires on it and they have a larger diameter.

 

How much larger are they going to be? Usually tires that are close to the same size won't throw off your speedo too much.

Check out this link: http://www.miata.net/garage/tirecalc.html

It'll tell you how far your speedo will be off with your new tires.

 

I'm going from 205/70r15s stock to 235/75r15s, so like 9.8%. My wife will complain about having to do algebra to figure out here milage. Is there a way to compensate in the internal workings of the car?

 

Unfortunately not. I'd just spend the $300 and get it calibrated.