The steering ratio, is the amount of degrees you have to turn the steering wheel, for the wheels to turn an amount of degrees. In motorcycles and bicycles, the steering ratio is always 1:1, because the steering wheel will always follow the wheel. x:y means that you have turn the steering wheel x degree(s), for the wheel(s) to turn y degree(s). In most passenger cars, the ratio is between 12:1 and 20:1. Example: If one complete turn of the steering wheel, 360 degrees, causes the wheels to turn 24 degrees, the ratio is then 360:24 = 15:1 (360/24=15).
A higher steering ratio means that you have to turn the steering wheel more, to get the wheels turning, but it will be easier to turn the steering wheel. A lower steering ratio means that you have to turn the steering wheel less, to get the wheels turning, but it will be harder to turn the steering wheel. Larger and heavier vehicles will often have a higher steering ratio, which will make the steering wheel easier to turn. If a truck had a low steering ratio, it would be very hard to turn the steering wheel. In normal and lighter cars, the wheels becomes easier to turn, so the steering ratio doesn't have to be as high. In race cars the ratio becomes extremely low, because you want the vehicle to respond a lot quicker than in normal cars. The steering wheel will also become a lot harder to turn.
A variable-ratio steering, is a system that uses different ratios on the ball, in a rack and pinion steering system. At the center of the rack, the space between the teeth are smaller and the space becomes larger as the pinion moves down the rack. In the middle of the rack you'll have a higher ratio and the ratio becomes lower as you turn the steering wheel towards lock. This makes the steering less sensitive, when the steering wheel is close to its center position and makes it harder for the driver to oversteer at high speeds. As you turn the steering wheel towards lock, the wheels begins to react more to your steering input.
A steering quickener is used to modify the steering ratio of factory installed steering system, which in turn modifies the response time and overall handling of vehicle. When a steering quickener is employed in an automobile, the driver of the automobile can turn the steering wheel a smaller degree when compared to a factory-installed steering system without a steering quickener, to turn the vehicle through same distance.
Another use of the term steering ratio is for the ratio between the theoretical turning radius based on ideal tire behavior and the actual turning radius based on real tire behavior. Values less than one, where the front wheel side slip is greater than the rear wheel side slip, are described as under-steering; equal to one as neutral steering; and greater than one as over-steering. Values less than zero, in which the front wheel must be turned opposite the direction of the curve due to much greater rear wheel side slip than front wheel have been described as counter-steering.