Жарыс автомобильдері үшін жақын қатынас қоробкасы

A close ratio transmission describes a motor vehicle transmission with a smaller than average difference between the gear ratios. They are most often used on sports cars in order to keep the engine in the power band.

Internal combustion engines found in passenger automobiles are capable of operating over a relatively wide range of speeds: idle to redline for petrol engines is approximately 700 to 6500 RPM or more; however, the power band, which is the optimum range of engine speeds considering fuel consumption, torque, and power output, is usually smaller. The automotive transmission is used to maintain the engine speed within the power band while operating the vehicle over a wide range of legal speeds. During acceleration, when the vehicle's speed increases to the point that the engine speed exceeds the speed at which maximum power is developed, the driver or transmission shifts to a higher gear (numerically lower ratio), which reduces engine speed, keeping it in its optimum power band, and allows continued acceleration. It is possible for the next higher gear ratio to be so much lower than the preceding ratio that upshifting lowers the engine speed excessively, resulting in the engine speed falling outside its "power band"; for maximum acceleration, the engine speed of an automobile should be kept in this power band. A wide ratio transmission requires the engine to operate over a greater range of engine speeds, but requires less shifting and allows a wider range of output (vehicle) speeds. High performance engines often are tuned for maximum power in an even more narrow range of operating speeds. A close ratio type of transmission is designed to allow an engine to remain in this relatively narrow range of operating speeds and generally are offered in sports cars, in which the driver can be expected to enjoy shifting often to keep the engine in its power band.

The 1967 Porsche 911 S was equipped with a 2.0 L flat six engine which produced at 6600/min and of torque at 5200/min. Using the Standard transmission gear ratios above, when the driver shifts from 2nd to 3rd gear at 6,600/min, the engine speed would fall to /min (which is 6600 × 1.32 / 1.78). In this case, shifting up to 3rd gear causes the engine speed to be slightly below the speed at which maximum torque is produced. By using a close ratio gearbox, such as the Hill Climb example above, shifting to 3rd gear would drop engine speed to /min (6600 × 1.55 / 2.00), which almost coincides with the maximum torque output of the engine. Likewise, the Nürburgring gearset above is also slightly numerically closer than the Standard gearset, making it more useful for sporting applications. However, the Nürburgring specification has a "taller" (numerically lower) 5th gear ratio than the Hill Climb gearbox, allowing for higher top speeds necessary for this faster racing circuit. The Standard gearset with its numerically lower 5th gear, will allow even lower engine speeds at highway speeds, thereby reducing engine noise and fuel consumption, but compromises acceleration performance at very high speeds.

Prior to the 1970s, manufacturers' manual transmissions generally had three or four gears. To meet requirements to maximize fuel economy, manufacturers began offering 5 and, in the 1990s, 6 speed manual transmissions. Likewise, 3 speed automatic transmissions were the norm until fairly recently, but now 6 , 7 , and 8 speed automatic transmissions are being offered. By reducing the spacing between ratios allowed by having more gears, a vehicle's engine speed can be kept in a narrow band. With a 5 speed transmission, the power range must be relatively wide, which requires compromising the engine's efficiency. With an 8 speed transmission, the power range can be kept relatively narrow, which allows the engineer to optimize engine efficiency at a particular engine speed, and the transmission attempts to keep the engine operating at that speed. (Engine efficiency improves greatly when the load on the engine is maximized; hence, automatic transmissions also upshift whenever possible in an attempt to lower the engine's speed as much as possible, which increases load and efficiency.) The recent introduction of continuously variable transmissions (CVTs) attempts to push this strategy to its logical conclusion. This allows a near infinite "number" of gear ratios, which this implies an infinitely close ratioed transmission. However, given that there are no gears or specific gear ratios, one would not really consider such a transmission close ratioed.