Stop in for a cup of coffee

OK here is a question for the guru's and if I don't get an answer here I will start a thread. I have a Motorhome with a 440 with a 727. The tires are 7.50 x 17.5 and I want to go to 7.50 x 20inch. What do I change in the transmission to have the correct speed register on the Speedometer?
I'm no guru.. but it would all depend on the circumference of the 20" vs the 17.5" tires. Keep overall height the same, no change necessary. If your going for larger tires, math comes inro play.