Rack-and-pinion steering works by using a pinion gear attached to the steering shaft that converts the rotational motion of the steering wheel to the lateral motion of the rack. On the ends of the rack are tie rods that connect to the tires and push them out or in.
The rack-and-pinion also acts as a gear reduction to reduce the amount of force needed to turn the car's wheels. This reduction is known as the steering ratio and is defined as the ratio between the amount the steering wheel is turned to the degrees the tires turn. Higher ratios mean more steering wheel movement is needed to turn the tires, although, in effect, less force is required.
Most vehicles require three to four revolutions before the tire is moved from full left to full right. Lighter cars, such as sports cars, use a lower ratio because they don't require as much force to turn, resulting in a much-appreciated, improved steering response.
Some cars include a variable ratio, where the tooth profile of the rack-and-pinion is different between the center and the outside of the pinion. This offers the combination of a quick response during the initial turn but a reduction in steering effort when the wheel is approaching full lock.