Gear ratio calculator.. I need one!

[short4stuff]

ok .. its for my 200x .. i want to get back to stock or close to gear ratio ..
but I want to figure it out with 18" tires and 20" tires...

anyone help me? .. stock gearing is 12/40 .. .I am running 13/40 with 21" tires..
anyone able to calc it, or have a place/program to do it?..

[Mr. Sandman]

OK HERE YOU GO,,,, 13/39 to mph (http://home.earthlink.net/~gellett/250r.htm)
You can thank KASEY for the calculator. He posted it in another thread. I just copied it to this one. Click on the link and at the bottom of the page is a list of other bikes & quads.