What is MPG, and how to improve yours

What's good MPG, by vehicle type

"Good" MPG varies significantly by vehicle class. A compact city car returning 50 mpg is acceptable; the same figure in an SUV would be exceptional. A more useful comparison is how your car's real-world MPG compares to similar vehicles.

Official figures vs real-world results

Official MPG figures are measured under the WLTP (Worldwide Harmonised Light Vehicle Test Procedure) testing cycle, introduced in 2018 and more realistic than the older NEDC cycle it replaced. Even so, real-world driving typically delivers 10–20% less than the WLTP figure, because test conditions don't capture:

• ❄️Cold weather: petrol engines use significantly more fuel when cold. A 5°C morning can reduce real-world mpg by 8–10% compared to a warm day.
• 🏙️Heavy urban traffic: the test cycle includes less stop-start driving than many real commutes.
• 🏎️Higher motorway speeds: WLTP tests up to 131mph but weights higher speeds more optimistically than most drivers experience.
• ❄️Accessories: air conditioning, rear defogger, heated seats, all add load and aren't consistently factored into test conditions.

How to measure your real MPG accurately

The most accurate method: fill the tank completely, reset your trip odometer, drive normally until near empty, fill the tank completely again. Divide the miles driven by the number of litres used (multiplied by 4.546 to convert to gallons). This gives your true mpg over real driving conditions.

Your on-board computer's MPG readout is useful for trends but is notoriously optimistic. Studies have found it overstates economy by 5–15% on average. Don't take it at face value.

The fastest ways to improve your MPG

• 🚗Smooth driving style: the highest-impact change. Anticipate, coast, avoid hard acceleration. See our driving style guide.
• ⚙️Higher gears at lower revs: shift up at 1,500–2,000 rpm. A car cruising at 50mph in 5th uses far less fuel than the same speed in 4th.
• 🔵Correct tyre pressure: a 10 PSI deficit costs ~3% fuel economy. Check monthly.
• 🧳Reduce load: remove roof boxes, clear the boot, don't run the tank overweight.
• 🔧Regular servicing: a clean air filter, fresh spark plugs, and correct engine oil all contribute to optimal combustion.

Servicing and its effect on MPG

A neglected service schedule has measurable and often underestimated effects on fuel economy. A clogged air filter forces the engine to work harder to draw air into the combustion chamber and can reduce fuel economy by 6–10% on its own. Worn spark plugs cause incomplete combustion, wasting fuel and reducing efficiency by 4–6%. Degraded engine oil increases internal friction throughout the drivetrain. Any of these alone produces a noticeable hit to real-world MPG; all of them together on a car long overdue for a service can add up to a 15–20% fuel economy deficit.

If your real-world MPG has declined noticeably over several months without a meaningful change in how or where you drive, consider whether a full service is due. A properly serviced engine running fresh oil, a clean air filter, and good spark plugs is one of the most cost-effective ways to recover lost economy without changing anything about your driving style.

A practical benchmark: if your current real-world fill-to-fill MPG is more than 10% below the figure you measured in the first year of owning the car (adjusting for similar driving conditions and season), it warrants investigation. The cause may be a routine service item, a minor sensor fault, or an early indication of something more significant — but it is worth checking rather than accepting the decline as inevitable.