Greetings, I have an old Mopar with a 360, 727, 8.75 out of a 64 Plymouth with 3.23 gears. The tach says about 2900 at about 65 mph. I don't have a working speedometer yet. Anyway, would simple math tell me have many r's if I change to a 2.91 or 2.76? (Experts, sorry if I don't have the numbers correct). Oh, the rear tires are 225x70x15. Thanks. I am thankful.