How to Improve Your Fuel Economy

Driving style, then speed, then trip planning, then weight and drag, then maintenance, in that order. The US Department of Energy puts aggressive driving at 10 to 40% more fuel in stop-and-go traffic and 15 to 30% at highway speeds. Every 5 mph above 50 mph costs roughly 7% more fuel. Combining short trips into one warm-engine journey can cut their fuel use by a third. Everything else is single digits.

The order matters because effort is finite. A driver who checks tyre pressure weekly but brakes hard for every junction is optimising the wrong thing. Fix the big items first; the small ones are worth doing only because they cost nothing.

Fuel is burned to accelerate, and much of it is then thrown away as heat in the brakes. Every hard stop wastes the fuel that built the speed. The technique is to look far ahead, lift off early when a red light or a queue is coming, and let the car slow on the engine. In a modern car the injectors shut off entirely on the overrun, so coasting in gear towards a stop uses no fuel at all.

Acceleration should be brisk but not hard, up to the target speed, then held. Lugging a petrol engine at very low revs in a high gear is not efficient either; the sweet spot for most is 1,500 to 2,500 rpm under moderate load. Cruise control helps on flat roads and hurts on hilly ones, where it accelerates uphill and brakes downhill, the opposite of what saves fuel.

The measured effect of switching from a hurried style to a smooth one is 10 to 20% for most drivers, which is worth more than every other item on this page combined. It costs nothing and takes about a week to become habit.

A lot, above about 55 mph. Aerodynamic drag rises with the square of speed, so the power needed to overcome it rises with the cube. A typical car uses about 15% more fuel at 70 mph than at 60, and 25 to 30% more at 80. Below 50 mph the effect flattens out and rolling resistance dominates, which is why economy is best somewhere between 45 and 55 mph in top gear.

On a 200 mile motorway journey, dropping from 75 to 65 mph adds about 20 minutes and saves roughly a fifth of the fuel. Whether that trade is worth it is a personal decision, but the numbers are not in doubt. A car rated at 40 mpg (US), 5.88 L/100km, will return something near 32 mpg at 80 mph and near 44 at 55.

Yes, modestly. The Department of Energy measures a 0.2% fuel penalty for every 1 psi of under-inflation averaged across four tyres, so tyres 10 psi down cost about 2%. Check monthly, cold, against the figure on the door pillar, not the maximum on the tyre. Under-inflated tyres also wear faster and grip less, which matters more than the fuel.

Weight costs about 1% per 100 pounds (45 kg) carried, more in city driving where the weight is accelerated repeatedly. Clearing a boot of tools and junk is worth a percent or two. Roof racks are worse than their weight suggests because they add drag: an empty roof rack costs 2 to 5% on the motorway, a loaded roof box 10 to 25%. Take them off when not in use.

Air conditioning costs 5 to 10% in hot weather at low speeds. Above about 45 mph, open windows cost about as much through drag, so use the air conditioning on the motorway and the windows in town. Heated seats and lights are negligible; the alternator load from a full set of accessories is under 1%.

Fewer than the marketing suggests. A clogged air filter on a modern fuel-injected engine reduces power but barely affects economy, because the engine management compensates. A failed oxygen sensor can cost 10 to 15%, and a sticking brake caliper or a dragging handbrake can cost more; both show up as a sudden drop in measured economy. Using the manufacturer's specified oil grade is worth 1 to 2% over a heavier one.

Fuel additives, magnets, vortex generators and the rest of the aftermarket economy industry have never survived independent testing. The US Environmental Protection Agency has tested over a hundred such devices and found none that worked as claimed. Premium fuel in a car designed for regular fuel does nothing for economy either.

Combine short errands into one journey with the engine warm throughout. A petrol engine in its first few minutes from cold can use double its warm consumption, and a two mile trip may never leave that phase. Three separate two mile trips use far more fuel than one six mile loop. Do the longest leg first, so the engine is fully warm for the stop-start part.

Avoid idling. A modern engine uses roughly 0.2 to 0.4 litres an hour at idle, and restarting costs less fuel than about ten seconds of idling. Cars with stop-start do this automatically; in cars without, switch off for anything longer than a minute. Route choice matters too: a slightly longer road at a steady 50 mph beats a shorter one with twenty junctions.

Measure, before and after, using the fill-to-fill method in the manual calculation guide. Trip computers are optimistic and drift, and a change of a few percent is invisible on them. Three tanks before the change and three after, over similar driving and weather, will show a real difference of 5% or more. Convert the results with the MPG to L/100km page if you want them in fuel-per-distance terms, which is the honest way to see a saving.

Expect the total from everything on this page to be 15 to 25% for a driver who was doing none of it, and near zero for a driver who already drives smoothly at sensible speeds with the roof rack off. The gains are real and they are finite.