Not sure if anyone else has posted this or is using it, but I found a tire size calculator. Just plug in the numbers for current tire size, then enter anticipated tire size and it will calculate diameter in inches & mm. It also gives you the speedometer error with the size tire you choose.
I thought this might be helpful (it was for me) since a lot of the questions I see here are on tire size differences.
http://www.anycalculator.com/tiresize.htm