What engine do you have???
There are two rough & ready checks that you can do if you don't mind getting underneath.
To determine 5th gear ratio, engage 5th - (engine not running).Then mark the propshaft (tippex works well). Turn the crank one full revolution. If the prop turns exactly one revolution it's 1:1. If it turns less than one complete rev then you have an overdrive 5th gear. It may be hard to turn over but you can push the car.....
If you jack it up to do this and don't have an LSD, it's difficult to get both wheels to turn the same amount without a helper!!
To check the diff, use the mark on the prop and see how far the wheels rotate for one revolution of the prop. If they turn a shade over 3 turns, then it's 3.07:1. If they turn more than 3.5 turns, then it's probably 3.7:1.
You need plenty of grunt to pull 30mph per 1000 revs, so unless the engine is special, I would suspect the revcounter. Can you get it calibrated with a multimeter?
