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?