If yer gonna change tire sizes your engine rpms are gonna change and your speedo will be off. http://www.moparregister.com/calc.php is a calculator that will tell you how much your speedo will be off with the new tires. It also has some good info on tires in general.

 

the bf goodrich website also has calculators for rpm/gearing/speedo/groundclearance....whatever else. both sites are handy

 

I use these 2 sites...

http://www.dakota-truck.net/TIRECALC/tirecalc.html

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

Hope this helps!