Close-ratio

A close-ratio transmission is a vehicle transmission in which the difference between the gear ratios of the gears is minimized relative to other transmissions used in similar vehicles. The use of "close" describes nearness rather than open-ness (i.e. open vs close).

There is no industry standardization between manufacturers as to what constitutes a close-ratio transmission. Therefore, a transmission that one manufacturer terms close-ratio may not necessarily be considered close-ratioed by another manufacturer.

This table compares the ratios of two BMW 323i vehicles from the 1980s, the first being the series transmission and the second being the optional close-ratio transmission.

In the series transmission, the 1st gear ratio is 4.73 times that of the 5th gear ratio, while in the close-ratio transmission the 1st gear ratio is 3.76 times that of the 5th gear ratio. Given that both transmissions have 5 gears, this is indicative of the latter transmission having gear ratios spaced closer together; hence, its being a close-ratio transmission.

Mathematically, this closeness can be represented by the cumulative average spacing between, or geometric average of, gears. For the above series transmission, each successive gear's ratio is 68% of that of the preceding gear (e.g. (1 / 4.73)^1/4 = 0.678). The close-ratio transmission has successive gear ratios 72% of that of the preceding gear. Thus, the close-ratio transmission above has each successive gear "closer" in numerical ratio to the preceding gear than that of the series transmission.

There is no specific figure that is used to denote whether the steps between gears constitute a normal or close-ratio transmission. Often, manufacturers use this term when offering a standard manual transmission and an optional, sportier transmission, one with closer ratios than the other, such as BMW above did. But this close ratio transmission is not necessarily closer in ratios than another manufacturer's normal manual transmission.

To maximize acceleration, the engine speed of an automobile must be kept near that where maximum power is developed. For this reason, vehicles utilize transmissions; as the vehicle's speed increases such that the engine speed exceeds the speed at which maximum power is developed, the driver shifts to a higher (numerically lower ratio) gear, which reduces engine speed and allows continued acceleration.

Internal combustion engines found in passenger automobiles are able to operate over a relatively wide range of speeds. A close-ratio type of transmission is designed to allow an engine to remain in a relatively narrow operating speed. Alternately, a wide-ratio transmission requires the engine to operate over a greater speed range, but requires less shifting and allows a wider range of output speeds. Close-ratio transmissions are generally offered in sports cars, in which the engine is tuned for maximum power in a narrow range of operating speeds and the driver can be expected to enjoy shifting often to keep the engine in its power band.

...
Wikipedia