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A shift light is a device used to tell a driver the optimum time to shift a vehicle. Commonly wired into a tachometer or a rev limiter, a shift light will illuminate a bright light—typically mounted in the driver's line of sight —when the engine has reached the shifting point. In some factory-installed configurations, the shift light will go out after a few seconds even if the transmission has not been shifted and will re-illuminate to once again remind the operator to shift the gears. Some factory designs provide two different shift light activation programs, one geared toward fuel mileage and the other programed for performance.
Many shift lights are attached directly to the tachometer so that the driver can verify the engine revolutions per minute (RPM) at the time a shift is made. Other placements position a shift light by itself on top of the dash or attached to a roll bar. Often, this placement is strictly driver preference and has no effect on the triggering of the light or the speed in which the gears are changed. Placing the shift light where the driver feels most comfortable and has the best vision will result in the best gear changes.
Testing has determined that light-emitting diode (LED) shift lights are the brightest as well as the quickest light. Although only a fraction of a second faster than a typical light bulb in illuminating, the LED light could potentially make the difference in a close race, such as drag racing, where winning and losing are separated only by tens of thousandths of a second. On-board computer systems evaluate a driver's performance to the speed in which he reacts to the flash of the shift light. Comparisons have proven that a driver can see and react to the illumination of the LED better than the traditional style light bulb, making the LED the light of choice in most competition vehicles.
In motorcycle racing, the shift light is especially helpful in determining shifting points due to the extreme vibration of the tachometer. A rider is often not able to look at the tachometer long enough to get a clear picture of what the gauge is reading; however, a flashing light is very easy to identify and shifting at the flash of the light provides a rider with a clear mental trigger. The shift light is also helpful to the rider when outside noise, such as wind and other motorcycle engines, make it difficult to hear the sound of the engine as it revs.