I know this has been posted b4 but also see alot of questions surfacing again sooooo .....

Our friends over in Miata land have a great Tire Size Calulator with allows quick comparisons of tire profile and speedometer error among other things...

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

 

My favorite is:

http://www.1010tires.com/TireSizeCal...?action=submit

It will let you compare multiple tires.

They also have a very good wheel offset calculator:

http://www.1010tires.com/WheelOffsetCalculator.asp

 

Here is one with the wheel and tire in one. It lets you change anything, lists all the stats, lets you compare, and even recommends equivalent tires.

http://www.rimsntires.com/rt_specs.jsp

It lets you set anything, I mean, you can have it show you a 375/25/12 tire and on a 4" wide rim. It is nuts.

 

Thanks -- That very helpful!