Gearbox and Diff

All gearboxes and differentials have various gear ratios. By changing a diffs crown wheel and pinion size, you would be changing your final drive, just as if you changed the size of your rear wheels diameters. The bigger the crown wheel and pinion or the more teeth they have, would shorten the ratio, and the less teeth on the gears would lengthen the final drive ratio. A diffs ratio can be determined by a simple calculation. Make a mark at TDC (top dead centre) on the propshaft and mark the position of one of the diffs sideshafts. For each 360 degree revolution made by the propshaft, count the turns made on the sideshaft in relation to the amount of degrees it moves. For example, if the sideshaft moves 3 full turns and the propshaft has rotated a full 360 degrees, then the ratio would be referred to as a 3,0:1 ratio. This determines the gear ratio of the differential.

‘Final drive’ is a term usually used to ascertain the final gearbox and diff combination ratio of a particular gear selected within the gearbox. Most 5 speed gearboxes have a 4^th gear that is usually a 1:1 ratio, meaning that the speed of the engines crankshaft or amount of rotations calculated by RPM (revolutions per minute), transferred through the gearboxes 4^th gear would in turn transfer the same amount of rotations through the propeller shaft to the diff. Each vehicles differential would have a standard ratio and when calculated together with a specific gearboxes gear selected, is what would determine the ‘final drive’ ratio.

All gearboxes and differentials have an assortment of ratios for various applications. If a vehicle needed more power or more top speed, then a diff gear change would be your easiest and most affordable way of altering the final drive of a vehicle. Wheel and tyre sizes are another way of changing the final drive in smaller increments in relation to the 360 degree full turn of a wheel.

Trucks and pulling vehicles for example require a shorter, more powerful final drive ratio while luxury cars require a longer and more fuel efficient ratio of gears. The ratios for Gearbox and Diff are endless and with a few simple calculations a good balance can be achieved.

If you are looking for a specific ratio more than often the entire diff can be purchased from your local auto scrap yard or used auto parts supplier at a fraction of the cost of a new unit from the vehicle manufacturers dealership. This may require some machining and fitment by a professional, but the best results can be achieved without burning a hole in your pocket.

For more information and a Free quote  CLICK HERE “Gearboxes / Diff / Specialist Engineering”  Call Centre :  0861 7777 22