Crawl Ratio

Crawl ratio is a term used in the automotive world to describe the highest

gear ratio A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission ...

that a vehicle is capable of. Note that gear ratio, also known as speed ratio, of a gear train is defined as the ratio of the angular velocity of the input gear to the angular velocity of the output gear, and thus a higher gear ratio implies a larger speed reduction, i.e. the input speed is reduced more at the output. The highest gear ratio is obtained at either first gear or reverse gear, but only first gear is typically taken into consideration while talking about crawl ratio. A potentially confusing terminology is that although a better crawl ratio is achieved by a higher gear ratio, it is common to refer to a better crawl ratio as “lower crawl ratio” rather than “higher crawl ratio” because it’s for driving at lower speeds. The crawl ratio is aptly named because when a vehicle is driven using the lowest gear (i.e. first gear), it moves the slowest (i.e. crawl speed) at a given engine

rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimension ...

, and thus produces the highest output

torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of t ...

(i.e. crawl torque) at the road

wheel A wheel is a circular component that is intended to rotate on an axle bearing. The wheel is one of the key components of the wheel and axle which is one of the six simple machines. Wheels, in conjunction with axles, allow heavy objects to be ...

s due to conservation of power. Since crawl ratio of a vehicle represents the total reduction of the engine speed until the road wheels, it is determined by combining the contributions of different elements on the entire

drive train A drivetrain (also frequently spelled as drive train or sometimes drive-train) is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the drivetrain is the components o ...

, including

transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission ** ...

and

differential Differential may refer to: Mathematics * Differential (mathematics) comprises multiple related meanings of the word, both in calculus and differential geometry, such as an infinitesimal change in the value of a function * Differential algebra * ...

, both of which typically introduce a certain amount of speed reduction. Since a lower crawl ratio (higher gear ratio) implies a larger output torque on the road wheels, it is desirable for vehicles that need to pull large loads, climb steep inclines, or drive over obstacles on the road or terrain, such as rocks, which is sometimes referred to as crawling over the rocks. Therefore, crawl ratios are most often discussed for large

SUV A sport utility vehicle (SUV) is a car classification that combines elements of road-going passenger cars with features from off-road vehicles, such as raised ground clearance and four-wheel drive. There is no commonly agreed-upon defini ...

truck A truck or lorry is a motor vehicle designed to transport cargo, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame constructi ...

s and

off-road vehicle An off-road vehicle, sometimes referred to as an overland or adventure vehicle, is considered to be any type of vehicle which is capable of driving on and off paved or gravel surface. It is generally characterized by having large tires with d ...

s. Note that tire size (or dimensions of the road wheels) does not affect the gear ratio of a vehicle, and thus using a different size tire on the same vehicle does not affect the torque on the road wheels or the crawl ratio. However, for a given engine speed and a gear ratio, the output force on the road wheels decreases as the tire size increases. A lower force in turn decreases the acceleration of rotating wheels. Therefore, the smallest tires that are still big enough to drive over obstacles perform better for a given crawl ratio. Crawl ratio can vary greatly among vehicles. Crawl ratios in the 60's (i.e. a gear ratio of 60:1) are quite capable; in the 80's, very respectable; in the 110's, impressive; and anything beyond the 130's is usually considered as the point of diminishing returns. {{DEFAULTSORT:Crawl Ratio Engineering ratios Mechanical power transmission