What is the calculation of the gear ratio?

Calculating the gear ratio for a car entails a lot of information. How high do you want the car engine to rev(turn-RPM) at 60 MPH? How much horsepower is the engine making and how much can be developed at the rear wheels?(I am assuming you're are referring to a rear wheel drive vehicle). How Much does the vehicle weigh? As you see that gear ratio takes an awful lot of information into it's calculation. For an auto manufacturer they have the engineers that can do this work. For the average "Joe" you will need to rely on experience car builders and custom designers. Good Luck.

Side Note: If you want an example- If you have your car4 in First gear(the lowest gear, the rear wheels turn at the same rate that the engine turns. That ration is 1:1.