districtqc- find out what rear ratio you have: Jack up one rear wheel, make a reference mark on the tire with a fixed object. Mark the driveshaft with a visible mark, relative to a fixed object. (for example, a line from the Ujoint straight to the rear housing snout.) Rotate the tire two full revolutions mark-to-mark x2, while counting the driveshaft rotations. The DS rotations is your ratio. If the shaft passes the fixed mark 3 times and stops 180* from the mark, it's a 3.50 rear, etc.
Once you know this data, you can easily calculate the rpm drop with different ratios, even without a tach.
Example- using bmbm40's #'s from his post, if it's a 3.50 rear and you consider a 3.25, then 350 over 325=1.077. (A 7.7% reduction in rpm across the board.) So whatever sound (rpm) you have now @ 55, that's the rpm @ 59 mph with the new gear. (55x1.077=59.24)
Or say you like the rpm @ 55, but want that rpm at 70. Then 55 over 70=.786 . If your rear is 3.50 now, then 3.50x.786=2.75 . You would have to drop to a 2.75:1 rear to accomplish that with a direct drive trans. (This is why OD was invented- the car will be a dog w/ 2.75 gears.)
