To get the corrected measurement for the tire size goto peagasus and it has the tire size calculator. Compare the 18" and the 20" if you have not gone radical on the sizes it should not make a difference the only reason I wrote this is that I have that 20" wheel and tire size from the factory, but here's the kicker my spare is a 18" wheel. If I remember right the numbers for both tires were equally the same. I'll post my 18" tire size tomorrow.
When you start the custom file process the gryphon puts in a bogus number for the tire size and defalts to 3.73 gears. This is about what I know about the custom file option.
Jack could probably tell you more.
Lars
