Anyone have a tool laying around to do so? I had one on my computer but lost it awhile back. I'm thinking about taking off the 225/60/16 and putting on the Nitto 245/50/16'* on from the Redrocket and I need to know how much the speedo is gonna be off and how many more feet I'll add per mile.

Try this one;

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

Cheers,

245/55-16 might be better.