Here is a good site to check the differences between tire sizes. The calculator even tells you how far off your speedometer will be off. By going from a 255/35/19 to a 285/35/19 at 60 MPH I'll really be running 61.9 MPH = 3.2%. http://www.miata.net/garage/tirecalc.html

 

Steve,

Thanks, that's a handy little tool... I built a spreadsheet some time ago to do something very similar. I'll have to add the speedometer variation calculation to my spreadsheet now.

 

I found this tire size calculator to be a little better.

http://www.rims-n-tires.com/rt_specs.jsp