Overdrive transmissions can save fuel, reduce emissions, and wear on engine components. They have a lower final drive ratio than non-overdrive transmissions, allowing for huge fuel savings. Overdrive can be turned off when hauling heavy loads to prevent damage to the transmission. Proper final drive ratio is important and installing overdrive on vehicles with root blowers or superchargers can increase fuel economy up to 100%.
Overdrive transmissions can be either automatic or manual gearboxes offering several benefits over a typical non-overdrive unit. By operating at a reduced gear ratio, overdrive transmissions actually spin at lower revs than the vehicle’s engine revs, saving fuel, wearing out engine components, and generating reduced emissions in most cases. Most vehicles found on the world’s roads are equipped with overdrive transmissions of one design or another. For the factory-equipped overdrive transmission, overdrive is commonly used as the standard drive gear with a disconnect or on/off switch mounted on the gear selector lever. On other vehicles equipped with aftermarket bolt-on overdrive units, the transmission is commonly controlled via a switch or handle mounted under the vehicle’s dash.
With a non-overdrive transmission, the final drive ratio is commonly 1:1, or one turn of the driveshaft for every full turn of the engine’s crankshaft. This allows an amount of power from the vehicle that still retains drivability on long trips. Any gear ratio lower would result in the engine running at very high revolutions per minute (RPM). In overdrive transmissions, the final drive ratio is typically 25 percent lower, with some final drive ratios as high as 2.67:1, or almost three turns of the driveshaft for every revolution of the engine’s crankshaft. This is how overdrive transmissions can produce huge fuel savings compared to other transmissions.
The advantage of being able to shut down overdrive transmissions is seen when pulling a trailer or hauling a heavy load. For overdrive to work properly, the engine must be running at a very low speed. This causes the engine to run outside of its power band or the most powerful engine speed. If you are pulling a heavy load with the overdrive feature on, the engine will continually move up and down while trying to gain power. If allowed to continue in this manner, overdrive transmissions will develop severe heating of the transmission fluid and components and eventually fail.
The proper final drive ratio for overdrive transmissions is obtained by factoring in weight, horsepower, and other vehicle factors such as rear gearing and power adders. On some vehicles equipped with root blowers or superchargers, installing overdrive transmissions can increase fuel economy by up to 100 percent. This is due to many factors, such as reducing fan heat when spinning the engine at slower speeds and tailoring shift points for optimum performance.
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