tire size chart

I like this one because it give you a visual of all the tire and rim dimensions. And you can compare two tires/rims next to each other:

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

This one is great for speedo calculations and speeds in gears. And being a B-body rallye dash is really cool:

http://vexer.com/68rt/speed.html