Fuel economy myths: seven tips that do not work

Fuel-saving advice is handed down through generations, and some of it retired along with the carburettor. Here are the seven most common, with an explanation of why each fails.

1. “Coasting in neutral saves fuel”

It does not, and it costs more.

On a modern engine, overrun cuts fuelling entirely — consumption at that moment is zero. In neutral the engine idles and burns fuel.

The car also handles worse in neutral, with no drive to adjust your line.

2. “Warm it up properly and it uses less”

It does not.

Idling is fuel at zero miles. An engine reaches operating temperature faster under moderate load than standing still.

The right way: start, wait for the idle to settle, and drive gently for the first few miles.

3. “Premium fuel is more economical”

Only where the handbook requires it.

Octane rating is resistance to detonation, not energy content. An engine designed for a lower rating cannot use a higher one and gains nothing.

The reverse fails too: using a lower rating than specified forces the module to retard timing — and the car loses both power and economy.

4. “Additives reduce consumption”

The effect is usually within measurement error.

Cleaning additives make sense as fuel system prevention. Promises of noticeable savings are marketing.

The confusion: if the injectors genuinely are fouled, cleaning helps. But then you are not saving fuel, you are fixing a fault.

5. “Fill up in the morning, the fuel is colder”

In practice, no.

Fuel in underground tanks holds a stable temperature regardless of time of day. The theoretical density difference is so small it disappears into the pump meter’s tolerance.

6. “Carry half a tank, less weight”

There is an effect, and it is tiny.

Half a tank of weight makes a fraction of a percent difference. For comparison, roof bars hurt aerodynamics several times more, and they do so even empty; covered in the fuel economy article.

The downside of the idea: running nearly empty draws sediment into the filter and encourages condensate; covered in the fuel filter article.

7. “Air conditioning always costs more than open windows”

It depends on speed.

In town at low speed, windows use less. On a motorway, open windows hurt aerodynamics more than the compressor costs in power.

Either way the difference is small — this is not the lever that decides anything.

What actually to check when consumption rises

A sudden rise is a diagnostic event, not the car ageing.

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

Engine temperature — an engine that never warms uses more permanently; covered in the thermostat article.

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

Tyre pressures — the cheapest and most common cause.

A sticking caliper — a hot disc after a gentle drive; covered in the pulling article.

In short

• Coasting in neutral costs more than engine braking.
• Idling to warm up is fuel at zero miles.
• Premium fuel works only where the manufacturer requires it.
• Morning refuelling and a half tank are within measurement error.
• 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.