In the automotive industry, gear ratio refers to the difference between the input and output speed of a differential or trans-axle. The purpose of the gear ratio is to give the best combination of performance and economy for a given engine size and vehicle weight. Some gear ratios are designed to operate outside of this condition, such as those used by hot rods in drag racing.
In the simplest terms, a gear ratio is the difference in size between two gears that have the same tooth spacing. One common ratio is the 4.10 gear ratio used in heavy-duty pickup trucks and drag-racing cars. This ratio is considered "low-geared"; for every revolution the larger gear (attached to the axles and wheels) makes, the small gear (attached to the drive shaft) makes 4.10 turns. This gear reduction gives increased power at the expense of top speed and fuel economy. An example of a "high-geared" ratio would be 2.62, where the large gear that drives the wheels turns once every time the drive shaft gear turns 2.62 times.
The effect of the gear ratio is to convert rotational energy--in this case coming from the drive shaft--to a desired level of energy at the wheels. The gear ratio does not increase horsepower; it merely changes how it is used. A lower ratio offers greater power while sacrificing top speed, whereas a higher ratio offers greater top speed with less power. Gear ratios also affect the vehicle's fuel economy. A higher gear ratio offers greater fuel economy on the highway, and a lower ratio will be less efficient. This is because the higher a gear ratio is, the fewer revolutions the engine has to make to achieve a desired speed. Since one of the things that affect fuel economy is engine RPM, fewer revolutions per minute use less fuel.
Gear ratios have existed since the first wooden gears were created. They were in use in applications such as windmills, where the weak rotational energy from the blades of the windmill was made usable by through slower speeds that turned a large grinding stone. Some uses predate even this. One of the simplest ancient gear ratios involves the well-bucket winch. A wooden cylinder with a handle was used to wind a rope and pull the bucket full of water up to the operator. This is an example of gear reduction, where ease of use was preferred to speed.
Gear ratio changes to a vehicle can change seemingly-unrelated aspects of vehicle operation. For example, changing the gear ratio of most vehicles will cause the speedometer to register inaccurately. The gear that drives the speedometer cable is sized according to the gear ratio. This gear must be replaced to ensure accuracy of the speedometer. One of the most frequently overlooked gear ratio changes occurs with changes to tire size. While a tire is not a gear, changes to its diameter have the same effect. Putting larger tires on a car effectively increases its gear ratio, costing power and gaining fuel economy, whereas smaller tires increase power but reduce fuel economy. Tire size changes also affect speedometer accuracy.
Changing the existing gear ratio can be highly beneficial if done to meet specific needs. For example, a large farm truck, or a truck used to pull a boat or trailer, can benefit from a lower gear ratio. The increased performance when stopping and starting, or going up hills, may easily outweigh the loss of fuel economy. The same is true for sports cars used in drag racing. A lower gear ratio gives more power to the wheels, at the cost of top speed; occasionally drag racers will benefit from a higher ratio, trading excessive wheel power for rotational speed. This depends on the weight of the car and the horsepower available. Many times the ideal ratio for a particular race car is not the ratio that the differential came with.
