I have 225x45x17 tires the stock tires were 185xx60x15 My question is how do I calculate how much my mileage is off...I believe it to be off by my estimation about 1 mile for every 10 traveled......Any help would be greatly apppreciated that is 1 mile less

George

 

You are 5.2% off compared to stock.

When your speedo reads 60mph, you are going 63.1mph.

And for every 100 miles on the odo you travel 105 actual miles.

 

Try this web page. Plug in your values and see the difference in MPH then calculate from there.

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

 

But...The odometer is 4.5% off on stock wheels. How does that factor into this calculation? I would think you'd have to drive 10 miles on an interstate and judge from the mile markers.