Gear is one of the most primitive components that are used to transmit rotational force to any other mechanical device or another gear. The primitive gears are made that were used in the olden days are made of wooden wheel with small pegs pointing out of it around the circumference of the wheel with regular gaps. Another wheel turns this wheel, both mounted on a shaft so that when one rotates the other also rotates due to the rotational force transmitted at the point of contact. But these types of gears not in use for the car engines or any mechanical device as the distance from the center to the point of contact keeps changing as the gears rotate. Thus the gear ratio keeps changing as the gear turns and hence output speed also keeps changing constantly. Thus it would result in the engine constantly accelerating and decelerating. Hence such gears are not in use now. Instead toothed profile gears called involutes are being used. This doesn't suffer from the problem mentioned above as with that of pegs. Thus it maintains the gear ratio.
Gear ratio is a number which is normally represented as a decimal fraction which can be defined as the ratio of the numbers of teeth on the meshing gears. It can be represented in the following way. Assuming there are two gears A and B the gear ratio is given as -
(Speed A * Number of teeth on A) = (Speed B * Number of teeth on B).
It is the reciprocal ratio of speeds of two gears. When a gear A is twice the diameter of gear B then the number of revolutions made to complete one full rotation of gear A is 2. This is because the circumference of the gear A is larger by a factor of 2. Thus increasing the time to revolve one complete rotation with respect to the point of contact of the gear B. Simply stated, the number of turns taken the input shaft to complete one full rotation of the output shaft defines the gear ratio.