Why Is My Car Using So Much Fuel?

Why fuel economy changes over time

Most drivers have a rough sense of what their car should return on a tank. When that figure drops noticeably — filling up more often, not reaching your usual range — it is worth investigating rather than accepting as normal. A car using significantly more fuel than it used to is usually either working harder than it should be, or burning fuel inefficiently.

The two broad categories are mechanical faults (something is wrong and the engine is compensating) and maintenance issues (something is simply due to be serviced or replaced). Both can be addressed once identified.

Tyre pressure and fuel economy

Under-inflated tyres are one of the most overlooked causes of poor fuel economy. A tyre running 10 PSI below its recommended pressure has a larger contact patch and more rolling resistance, meaning the engine has to work harder to maintain speed. The effect is measurable — correctly inflated tyres can improve fuel economy by 1 to 2 miles per gallon.

Check pressures monthly with a gauge, not by looking at the tyre. The correct pressures are on a sticker inside the driver's door frame or in the handbook — not on the tyre sidewall, which shows maximum pressure.

Engine maintenance causes

A clogged air filter restricts airflow into the engine, forcing it to use more fuel to maintain power. Air filters are typically replaced at every full or major service but can block earlier if you drive on dusty roads. A spark plug that is worn, fouled, or gapped incorrectly causes incomplete combustion — the engine burns more fuel to achieve the same output.

Oxygen sensors measure the exhaust gas composition and tell the ECU how much fuel to inject. A faulty oxygen sensor causes the engine to run rich (too much fuel) or lean (too little fuel). Running rich wastes fuel and can damage the catalytic converter over time. A diagnostic scan will usually reveal a faulty oxygen sensor through a stored fault code.

Maintenance items that affect fuel economy:

• Air filter — restricts airflow when blocked
• Spark plugs — incomplete combustion when worn
• Oxygen sensors — cause rich or lean running when faulty
• Fuel injectors — spray poorly when dirty, causing inefficient burn
• Coolant temperature sensor — causes rich fuelling if it reports a cold engine permanently
• Tyre pressures — rolling resistance increases with under-inflation

Sticking brakes and increased load

A sticking calliper or handbrake cable can apply partial braking continuously without you noticing. The engine then has to work harder to maintain speed, burning more fuel. A clue: a car with a sticking calliper will often pull slightly to one side and the affected wheel may be noticeably hotter than the others after a journey.

Roof bars, roof boxes, or heavily loaded boots all increase aerodynamic drag and fuel consumption. Remove roof bars when not in use — even empty bars add meaningful drag at motorway speeds.

When to get it diagnosed

If your fuel economy has dropped suddenly rather than gradually, a diagnostic check is worthwhile. Fault codes for oxygen sensors, mass airflow sensors, and injector faults are often stored even when no warning light has appeared. We can also carry out a visual inspection of brakes and tyres to rule out mechanical drag. Book a check with The Car Guys on 01527 336608 or online.