Fuel economy: what actually works and what is a myth

Fuel economy advice is dominated by tips that either do nothing or work in reverse. Here it is in order of real payback.

What works best

Driving style. The largest factor of all, and it is free.

Specifically:

• Smooth acceleration — not slow, but without snapping the throttle open.
• Reading the traffic — lifting off early instead of braking at the last moment. Every brake application is fuel converted into heat.
• Steady speed instead of accelerate-brake.
• Changing up early on a manual.

Tyre pressures. Under-inflation causes noticeable overconsumption while also ruining tyres and lengthening stopping distances. Check monthly, cold; covered in the tyre wear article.

Motorway speed. Air resistance rises non-linearly: the consumption difference between 70 and 80 mph is far larger than the difference in journey time.

Remove what you are not using. Roof bars and boxes are the main culprits: they ruin aerodynamics, and they do so even when empty. Take them off when not needed.

What works moderately

Timely servicing. A blocked air filter, worn plugs, old oil — each is small alone, together they are noticeable; covered in the air filter article.

The correct oil viscosity — to the manufacturer’s approval.

Not carrying half a tank you do not need — a small effect, but real.

What does not work

Idling to warm up “for economy”. The reverse: idling is fuel at zero miles. An engine warms faster under moderate load.

Coasting in neutral downhill. On a modern engine, overrun cuts fuel entirely. In neutral the engine idles and burns fuel. So neutral costs more, and it is less safe as well.

“Economy” additives — the claimed effect is usually within measurement error.

A performance air filter — covered in the air filter article.

Premium fuel in an engine that does not require it — the engine will not use more octane than it was designed for.

Air conditioning or open windows

The classic question with a nuance.

In town at low speed, open windows use less.

On the motorway, open windows hurt aerodynamics more than the compressor costs in power. Here air conditioning wins.

In practice: the difference is small either way; this is not the lever that decides anything.

What to watch in diagnostics

Here numbers replace guesswork.

Instantaneous and average consumption — the main teaching tool: you see exactly what acceleration costs and what speed costs.

Fuel trims — a steady positive value means the module is adding fuel because of a fault; mechanics in the fuel trim article.

Engine temperature — an engine that never warms uses more, permanently.

Ignition timing — timing retarded by detonation means losing both power and economy.

A sudden rise in consumption with no change in driving is a diagnostic event, not “the car getting old”.

In short

• Driving style is the biggest factor and it is free.
• Check tyre pressures monthly, cold.
• Roof bars hurt aerodynamics even when empty.
• Coasting in neutral costs more than engine braking.
• A sudden rise in consumption is diagnostics, not ageing.

Download the app — consumption, fuel trims, and temperature are read with an ELM327 adapter over Bluetooth SPP on Android.