Well, doing math with the GPS and the speedometer, this takes the actual rear tire size into the formula. You don't even have to figure it in, which is what you may be missing with the .74 (2:76 + 3:73) number. The difference between the orig 7.00 X 14 versus the new diameter tire. These were the readings driving the car:

GPS mph divided by speedometer reading = reduction required
40 divided by 60 = .66666667
50 divided by 75 = .66666667
60 divided by 90 = .66666667

.67 should put it spot on...