Mind the gap when it comes to gear ratios


The gaps between intermediate gears will depend on the size and power of the engines

Photo credit: Shutterstock

Do all cars have roughly the same gear ratios and the same gaps between gears?

Or do they vary, and why?

They all have the same objective – to enable the vehicle to do its job as efficiently as possible. But the vehicles themselves and the jobs they are designed to do vary – sometimes by big amounts and in many different ways—so the ratios and gaps are tailored accordingly.

What every design has to achieve is at least one gear low enough to get the vehicle moving from a standstill, easily (without over-revving or slipping the clutch) even when fully laden and pointing uphill. And one gear high enough to enable it to cruise at a reasonable ambient speed, at a rev level that does not stress the engine. The gears in between enable a progressive transition from first start to cruising speed.

The gaps between those intermediate gears will depend on the size and power of the engines, the most efficient rev band of each engine design, the likely loads they will have to carry (including the vehicle’s weight), and the performance/economy preferences of their owners. All of those are considered in the manufacturer’s choice of ratios and gaps. Most modern vehicles have five speeds, but some older models have as few as three and some big trucks have more than 20! Constant Variable Transmissions (CVTs) have none— or an infinite number, depending on which way you look at it; their “gearing” ratios slide up and down a spinning cone, rather than in “steps” from one side of the gear cog to another.

The over-riding principle of all of them is that gearing ratios are a trade-off between power and speed. The lower the gear, the more power (torque, turning force) is available to the wheels from a given engine.

Higher gears reduce the power but enable cruising at lower (more economical and comfortable) engine revs, and raise the potential top speed…if engine power and road conditions enable maximum revs to be reached in top gear.

The usual equation is to allow the driver to keep the engine revs in their most responsive rev band – in cars that is commonly between about 2,000 and about 4,000 revs. To enable a clear explanation of the principle, let’s take 3,000 revs as the middle of that band. Gaps will normally be set so that from that midpoint, the driver can change up one gear or down one gear and still be in the “responsive” 2k to 4k band and thus neither labour nor stress the engine.

Don’t get fixated on those exact numbers, because they will vary. Diesel engines are generally lower revving than petrol engines. Big trucks can carry more than double their own weight in cargo so need more grunt (in “torque”) at the bottom end than cars, which rarely carry even half their own weight and want more performance (in “horsepower”) at the top end. Ratios and gaps are adjusted to deliver those requirements.

The gaps between gears can of course be reduced by having more gears, but that would make them more expensive and fussier to drive.

Five gears is the usual compromise and it can readily comply with the gap principles in ordinary cars; a bit harder for workhorse vehicles (for example, pick-ups) because they need a lower first gear so they must have a bigger gap somewhere in the chain or a lower top speed to compensate.

The solution to a bigger gap is having to rev a bit higher before changing up.