Why Is My Car Using More Fuel Than It Used To?

You are filling up more often than you used to and nothing obvious has changed. No warning light, no strange noise, the car drives the same.

Why is my car using more fuel is one of those questions where the answer is usually mundane, cheap and cumulative. Several small things each cost a few percent, and together they add up to a noticeable difference at the pump.

Measure it before you chase it

The trip computer is an estimate and it drifts. Before assuming something is wrong, get one real number.

Fill to the click, note the odometer, drive normally for a full tank, then fill to the click again and divide. That is your actual consumption, and it is the only figure worth comparing against.

Do it twice if you can, because a single tank can be skewed by an unusual week. Two consistent tanks is evidence; one is an anecdote.

The cheap causes, which are most of them

Rule these out before paying anyone to look, because between them they cover the majority of gradual economy loss.

• Tyre pressures. Underinflated tyres deform more and cost fuel directly, and they drop naturally over months without ever looking flat.
• A roof rack or box left on. The aerodynamic penalty at motorway speed is far larger than people expect, and an empty rack costs almost as much as a full one.
• Weight you are carrying without noticing. A boot that has quietly become storage is dead weight on every journey.
• A clogged air filter, which is inexpensive and often years overdue.
• Cold weather and short journeys. An engine uses noticeably more fuel until it reaches temperature, so winter economy is genuinely worse and it is not a fault.
• Air conditioning used constantly at low speed, which is a real if modest cost.

Tyre pressure is the one worth doing today. It is free, it takes five minutes, and it is the single most common cause on this list.

Has your driving changed without you noticing?

Before blaming the car, look at the pattern. A change of job, a new school run or a house move can shift you from steady runs to stop-start traffic without any conscious change in how you drive.

Short journeys are dramatically more expensive per mile than long ones, because the engine spends most of its time cold and inefficient. The same car, driven the same way, can differ substantially between a motorway-commute month and a school-run month.

Speed matters too, and more than most people think. Aerodynamic drag rises sharply with speed, so a steady seventy costs meaningfully more than a steady sixty over a long journey.

The mechanical causes worth paying to investigate

If the cheap list is clean and the number is still worse, these are the usual suspects.

• Oxygen or lambda sensors reporting inaccurately, so the engine runs richer than it needs to. A classic cause of gradual, silent economy loss.
• Ignition components past their best, giving incomplete combustion.
• A sticking brake, which drags constantly and often shows as one wheel much hotter than the others after a drive.
• Wheel alignment out, so the car is scrubbing rather than rolling. This normally shows in tyre wear first.
• A thermostat stuck open, so the engine never reaches its proper operating temperature and runs rich permanently.
• On a diesel, a particulate filter attempting regeneration frequently because of short journeys.

The thermostat one is worth flagging because it produces almost no other symptom beyond a temperature gauge that reads a little low and a heater that never gets properly hot. Plenty of people live with it for years and simply accept worse economy.

Fuel economy and the engine management light

A steady engine management light and worse economy together is a strong combination, because many of the sensor faults that harm consumption are exactly what the light exists to report.

The car has already recorded a code. Reading it is quick and converts guesswork into a specific answer, which is a much better starting point than replacing parts in order of price.

What is realistic to get back

Diesels and electric cars have their own version of this

The list above is written for a petrol car, and the other two drivetrains fail in their own characteristic ways.

On a diesel, the most common cause of gradually worse economy is a particulate filter attempting regeneration too often. Regeneration burns off accumulated soot and it needs a sustained run at temperature to complete. A car doing short journeys keeps starting the process and keeps interrupting it, which burns extra fuel each time and achieves nothing. The fix is usually a change of journey pattern rather than a part.

On an electric car the same question appears as reduced range, and the causes rhyme: soft tyres, extra weight, a roof box, cold weather and more short journeys. The difference is that cabin heating is a much larger share of the total in an electric car, so a cold winter hits range harder than it hits a petrol car's economy. Preconditioning while still plugged in is the equivalent of the roof-box fix, and it is free.

Worth setting expectations honestly. Correcting pressures, removing a roof box and a clean air filter will get you a real improvement, and it is the cheapest few percent you will ever buy.

What none of it does is beat the official figures, which are produced under laboratory conditions no real journey reproduces. Comparing against your own past average is meaningful. Comparing against the brochure is not.

And if consumption changed suddenly rather than gradually, treat that differently. A sudden step points at a specific fault with a date attached, and that is far easier to find than a slow drift caused by five small things at once.