If you do decide to do that go with that company. They shipped the adapters very quickly and they work great. Buy the adapters off their ebay page though. It is actually cheaper that way than buying from their actual website.

It ends up being $159 for a set including shipping. We ordered them Saturday. They shipped Monday and we recieved them Thursday.

 

Nice looking rims.

 

This is a great tool to check tire size and speedo calibration

http://www.toolmandesign.com/tireapps/tirecalc.html

 

I doubt it is off much. Stick your gps in there and check it. My speedometer has never been correct and I have owned the car since 1970. When I was a kid I would have a friend follow me with their dad's buick and check speed then keep that in mind. Now I just run my gps or don't worry about it.
Dan

 

Here is another site I've used to play around with tire sizes. http://miata.net/garage/tirecalc.html

 

The only problem with these programs is they only do same size tires all around. I have2 different sizes.

 

If you have different sized tires, input the rears only. Fronts will not have any impact on the speedo reading.

 

Ok. Then it says my speedo is .5% slow. It said when the speedo was going 60 I was really going 60.3 Not a big difference at all.