What Actually Affects Your Car's Fuel Efficiency

Fuel efficiency attracts more folklore than any other aspect of car ownership. Some of the standard advice is genuinely effective, some is marginal, and a good deal of it is simply wrong. Here is what the physics supports.

The biggest factor is you

Driving style dominates everything else, and the gap between an efficient driver and an inefficient one in the same car is routinely twenty to thirty percent. No modification or additive comes close.

The mechanism is simple: every time you accelerate, you convert fuel into kinetic energy, and every time you brake, you throw that energy away as heat. Efficient driving is mostly about not creating speed you are about to discard.

• Look far enough ahead to lift off early rather than braking late. Approaching a red light at a speed that lets it turn green before you arrive uses dramatically less fuel than accelerating to it and stopping.
• Accelerate briskly but smoothly to your target speed, then hold it. Crawling away in too low a gear is not economical; labouring the engine is its own inefficiency.
• Shift up earlier than instinct suggests on a manual. Engines are most efficient at moderate load and low revs.
• Maintain a steady speed. Constant small variations in throttle cost meaningfully more than a fixed cruise.

Speed matters more than most people expect

Aerodynamic drag rises with the square of speed, so the power required to overcome it rises with the cube. In practice, that means the fuel cost of the last twenty kilometres per hour is disproportionate: most cars are noticeably more efficient at 80 km/h than at 110.

This is also why roof boxes and roof racks are expensive to leave fitted. An empty roof rack costs real fuel every kilometre you carry it. Remove it when you are not using it.

Tyre pressure is the cheapest real gain

Underinflated tyres deform more as they roll, and that deformation consumes energy. Chronic underinflation across all four tyres measurably increases consumption, aside from being a safety problem and ruining the tyres.

Check pressures every fortnight when cold, using the figure on the door frame sticker. This is genuinely the highest-return five minutes available, and it costs nothing.

What does not follow is that overinflating helps. It reduces rolling resistance slightly, but at the cost of a smaller contact patch, longer braking distances and uneven wear. That is a poor trade.

Weight, and the myth of the empty boot

Weight matters, but it matters much less in steady cruising than in stop-start driving, because its cost is mostly in acceleration. Removing fifty kilograms of accumulated junk from the boot produces a small improvement in city driving and almost none on the highway.

Worth doing, but it is not the lever people imagine. The frequently repeated advice to run the tank half empty to save weight is not worth acting on: fuel weight is modest relative to the car, and the inconvenience and risk of running low outweigh the gain.

Air conditioning

The compressor is driven by the engine and consumes real power, so running the AC does increase consumption, particularly on small engines and in slow traffic.

The usual counter-advice, open the windows instead, holds only at low speed. Above roughly 60 to 80 km/h, the aerodynamic penalty of open windows generally exceeds the compressor load, so on the highway the AC is the more efficient choice. In town, windows win.

Where you can save reliably is by not asking the AC to do unnecessary work: park in shade, use a windscreen shade, and vent the hot air out for a few seconds before setting off rather than cooling a sixty-degree cabin from the inside.

Maintenance items that genuinely matter

A clogged air filter restricts airflow and makes the engine work harder to breathe. Worn spark plugs cause incomplete combustion. Dragging brakes, from a sticking caliper, waste energy continuously and can be surprisingly costly. Poor wheel alignment makes tyres scrub sideways as they roll. Old, degraded engine oil increases internal friction.

None of these individually transforms consumption, but together they explain most of the gap between a well-maintained car and a neglected one of the same model.

Short trips are the hidden expense

An engine is at its least efficient when cold, because fuel does not vaporise well and the management system deliberately runs a richer mixture until things warm up. A five-kilometre trip may spend most of its distance in that state.

Combining errands into a single longer journey rather than several short ones makes a genuine difference to both consumption and engine wear. This is also the single strongest argument against buying a diesel for short urban trips, since diesels take longer to warm and their particulate filters need sustained running to regenerate.

Idling

A modern engine uses essentially no fuel when coasting in gear but continues to consume while idling. If you will be stationary for more than a minute or so, switching off saves fuel. Cars with automatic start-stop do this for you, which is exactly why the feature exists.

The related myth is that starting uses more fuel than idling. That was arguably true of carburettor engines decades ago. It is not true of modern fuel injection, where the break-even point is a matter of seconds.

What does not work

Fuel additives that promise significant economy gains do not deliver them. Detergent additives can help clean injectors on a neglected engine, which may restore lost efficiency, but that is recovering ground rather than gaining it. Magnets, vortex generators for the intake, and similar devices have no plausible mechanism.

Premium high-octane fuel does not improve economy in an engine not designed to exploit it. Octane rating describes resistance to knock, not energy content. If your manual specifies regular, higher octane buys nothing.

Measure properly

Trip computers are convenient but often optimistic. To measure honestly, fill to the same click at the same pump, note the odometer, drive normally for a full tank, and fill again. Divide distance by litres. Do it over several tanks, since a single measurement is heavily influenced by the mix of driving in it.