The simplest and easiest way to find out what your gear ratio is to jack up the rear of the car, I recommend putting it on jackstands for safety reasons, get some white out and mark the driveshaft. Spin one of the wheels slowly one time by hand and count how many times the mark you made on the driveshaft goes around. 3.5 turns would be 3.55 gears, 3 3/4 times would be 3.73 gears, 4 times would be 4.10s, etc.
You could also pop the diff cover and look for markings on the gear, but then you have to deal with the nasty gear oil and re-sealing the cover. I'm not sure about all the different brands of gears, but the Ford racing gears are stamped with their gear ratio.
If you car doesn't have an aftermarket gear in it I would recommend putting one in it- the best seat of the pants improvement you can make for little money.