Gear ratio question
Well, this is not rocket science. There ARE online calculators, but---
easiest way is to beg or borrow a GPS
Measure the tires all the way around, then do some math
Figuring the TRUE rear ratio is easy, first---do you know if you have a limited slip rear? If not jack up ONE rear tire, front tires blocked, put the car in neutral Can you turn one tire? If so, turn the tire TWO turns, and count the driveshaft
If you cannot turn the single tire, you have limited slip. So now, jack up BOTH rear tires, and turn one of 'em JUST ONE turn, and count the driveshaft
According to this:
http://www.race-cars.net/calculators/mph_calculator.html
3500RPM, 28" tire, 3.91 rear is 75 mph
According to that same calculator, you would indeed have a 4.56 gear
According to this:
http://www.gordon-glasgow.org/tirecalc.html
your tire size is indeed 28" (27.99)