Just wondering if there's a way to tell what gears I have without cracking open the housing. I was wondering if theres a certain RPM range the car will be at X MPH per gear ratio for a mostly stock 88 5.0 mustang. Thanks for any help.
If you know the rear tire diameter (and by that, revolutions per mile) and the number of teeth on the speedometer gear (easy to get to) you have, you can ballpark it by using the formulas on here:
Speedometers and tachometers aren't always super accurate, but even then to figure out what your rear end ratio is by using them, you'd also need to know gear ratios in your transmission (which varied some over the years).
There is always the axle tag method, if you know for sure the gears are stock. A search on the 'net could find you good results on serial numbers for different gear sets.
Rear End Ratio = Tire Height (in.) x 0.0000496 x Engine RPM / Trans. Gear Ratio x 60 / MPH
Example:
26.0" x 0.0000496 x 2200rpm / 0.68 x 60 / 70mph = 3.57 (a 3.55 rear end)
Of course this would assume that your speedometer is accurate. If your rear gears are not stock, then you speedo could be way off.
The best method is to look for the axle tag, but that could be left there even after a gear swap so...
The second best method is to turn the rear wheels, and count the number of times the driveshaft rotates. You could turn it once, but it's hard to tell the difference between 3.1 revolutions and 3.3 revolutions, or between 3.3 revolutions and 3.5 revolutions when you're looking at it (trust me I know). It's best to mark both the wheel and the driveshaft, and have a helper turn the wheel around exactly 10 times while you count the number of times the driveshaft rotates. Divide that number by 10.