Doctor Diesel

Back to Basics: Gear ratios

Audi S tronic 7-GangIn past times, road testers often made comments like “there’s something of a gap between third and fourth that maybe makes overtaking 50mph traffic more difficult than it should be”, and we maybe wonder how the designers and manufacturers could arrive at such ill-chosen ratios. Even now, it may sometimes be evident that sixth gear seems a little too high, such that motorway overtaking may necessitate a drop down to fifth gear to make good progress.

Taking it for granted that we understand that it’s all achieved by juggling the number of teeth in the various meshing gears, how are the ratios chosen for the gearbox internals and for the final drive, and why are they sometimes apparently imperfect? There is of course a mathematical way of choosing gear ratios, by matching them to the engine’s power and torque characteristics, as determined on an engine dynamometer. Ideally you would want the gear spacing to be such that when you change up a gear the engine speed falls into a good meaty part of the torque curve in the next higher gear. With this result, you should get close to optimum acceleration and also hopefully optimum economy, as the engine’s peak torque speed is also the peak economy speed. With twin clutch automatic gearboxes, like VW Group’s DSG/S tronic, and Ford’s PowerShift, each of the two gear clusters has a different final drive ratio, which just makes the calculations a touch more difficult.

Accepting that most open road motoring is generally done in the two highest gears, ratios will generally be chosen more for best fuel economy and refinement, whilst in the lower gears, and maybe all the ratios in more sporting cars, the spacing will be more in line with achieving maximum acceleration and top speed. For a specific model though, variants with bigger and more powerful engines usually run with a higher top gear. Putting aside gear ratios and final drive ratios, which are mere numbers, the key figures for a driver are the mph per 1,000rpm in each gear: a typical five-speed transmission 1.6-litre diesel might offer 5.4, 10.5, 17.1, 24.8, and 32.3mph per/1,000rpm, while a 2.0-litre diesel might offer six ratios of 5.8, 11.2, 17.4, 25.1, 31.8, and 38.1mph per/1,000rpm. There’s little difference here in the lower gear ratios, but the availability of an extra ratio allows the 2.0-litre car to cruise at 70mph at just 1,840rpm, whilst at 70mph, the 1.6-litre unit spins at 2,170rpm, at which it may be less economical than the bigger engine.

But the EC fuel economy figures to which manufacturers are tied, and which contain very little high speed driving, will almost always give better economy results for a smaller engine. But we have to bear in mind that the demand for manufacturers to produce low EC test figures, different terrain, variable legal limits, and alternate driving styles all have to be accommodated, and the ratios specified with manual transmissions will not necessarily suit all drivers.

CVT (continuously variable transmission) offers infinitely variable ratios, and computerised transmission management theoretically adjusts to offer a perfect ratio for maximum acceleration and the best high ratio for economy at steady speed cruising, with infinite variables between these two extremes according to power demanded and engine loads. In practice, there are shortcomings in efficiency and complexity with CVT, and the motor industry is trending more towards computerised twin-clutch transmissions and conventional torque converter automatic transmissions with up to nine speeds, both of which again aim to offer automatic selection of near-ideal ratios for any given driving situation, and fuel economy close to, sometimes better than, manual transmissions, where anything beyond six ratios is probably inconceivable.

Many drivers change from standard wheel and tyre sizes, usually upgrading to bigger diameter wheels, which demands expert advice to choose the correct tyre profile and size to maintain the correct gearing, and speedometer accuracy.

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