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Are the fifth and sixth gears in my 2016 Mazda BT-50 overdrive gears?

Asked by Terry

I have a 2016 Mazda BT-50, are both fifth and sixth gear in the manual gearbox an overdrive gear?

Answered by CarsGuide

28 Oct 2020 David Morley

To put your question into context, an overdrive gear is one where the output shaft of the gearbox spins faster than the input shaft. Or, put another way, a gear in which the car’s propeller-shaft is spinning faster than the engine’s crankshaft. This means the drive wheels can be spun faster (for more road speed) without making the engine rev too hard. Overdrive gearboxes have been common for many years now, typically when five-speed transmissions replaced four-speed units. Those earlier four-speeds generally had a 1:1 ratio on their fourth (top) gear which means the output shaft (and propellor shaft) spun at exactly the same speed as the input shaft (or engine).

Many manufacturers have now, of course, switched to six-speed transmissions and some do, in fact, use that opportunity to fit an overdrive fifth and sixth gear. But in the case of your Mazda, only sixth gear is overdriven (with a ratio of 0.794:1) while fifth gear takes the place of a traditional fourth gear by being 1:1. That gives Mazda the chance to make fourth gear a little lower and tighten up the gaps between all the gears to eliminate any dips in the power delivery. Ultimately, of course, how fast the engine revs at a given road-speed is also down to the differential (or final-drive) ratio fitted, and the diameter of the wheel and tyre package.

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