What MPG should my car do? Making sense of official fuel economy figures

Fuel economy is one of the most-quoted and least-understood numbers in car buying. Every manufacturer brochure lists an official "combined" MPG, every used-car ad quotes a number, and yet drivers almost universally find that the figure they actually see on their own trip computer sits a long way below the advertised one. Understanding where the figures come from — and what your car should realistically be doing — helps you budget more accurately and spot early warning signs that something's wrong.

Where MPG figures come from

Every new car sold in the UK has to be tested in a laboratory to a standardised protocol before it can be put on sale. The results are submitted to the Vehicle Certification Agency (VCA), which publishes them in an annual open dataset that powers our checker. Currently all new cars are tested under the Worldwide Harmonised Light Vehicle Test Procedure (WLTP), which replaced the much-derided NEDC protocol for new cars from September 2017 onwards.

Under WLTP, cars are run on a rolling road across four distinct phases — low speed, medium speed, high speed, and extra high — totalling about 30 minutes of driving and 14 miles. A weighted average produces the familiar "combined" MPG figure, and separate numbers are quoted for the urban (low-speed, stop-start) and extra-urban (higher-speed, motorway-like) phases. The test is run with the doors closed, air-con off, headlights off, no roof rack, and a trained driver following a specific accelerator trace. Your real-world conditions rarely look like that.

Why your real-world MPG falls short

Most drivers see 15–25% worse fuel economy than the WLTP combined figure suggests. The gap gets bigger under NEDC, which was around 30% optimistic on average. The reasons are mundane but cumulative:

  • Driving style. Hard acceleration, high cruising speeds, short journeys from cold — each knocks 5-15% off.
  • Cold starts. An engine burns extra fuel for the first 3–5 miles until it reaches operating temperature. A lot of British driving is made up of short trips that never let the engine warm up properly.
  • Air conditioning and heating. Running the A/C can cost 5-10%, and in winter a heated rear window, heated seats, and wipers drag the alternator harder.
  • Tyre pressures. Under-inflated by 10 PSI can cost around 2% — not dramatic, but a free fix worth doing.
  • Roof racks, bike racks, roof boxes. At motorway speeds, aerodynamic drag can cost 15–25% even when empty.
  • Heavy loads and towing. Towing a caravan or trailer typically halves MPG on long motorway runs.
  • Traffic. Constant stop-start on an urban commute can drag combined figures down to nearly half what the brochure promises.

What counts as "normal" MPG for your car?

A reasonable rule of thumb: expect to achieve 75–85% of the official combined figure in mixed driving, and 60–70% of it in purely urban stop-start traffic. If you're regularly seeing less than 60% of the official figure, something isn't right:

  • Dragging brakes — a stuck caliper keeps one wheel partly applied, typically causing a noticeable pull to one side and an odd warm smell after a drive.
  • Blocked air filter — a £15 service item often overlooked during a DIY oil change.
  • Failing oxygen sensor or MAF sensor — makes the engine run richer than it should, tanks fuel economy, and usually but not always triggers a check engine light.
  • Incorrect tyre sizes — a previous owner fitted larger-profile tyres, making the odometer under-read distance and inflating the apparent fuel use.
  • Diesel particulate filter problems — a DPF stuck in regeneration mode can destroy fuel economy on short-trip drivers.

Measuring your own MPG accurately

Trip computers on modern cars are typically 5–15% optimistic. If you want a truly accurate figure, use the old-school method: fill the tank to the first click of the pump, reset your trip meter, drive until you're about a quarter tank remaining, refill to the first click again (same pump if possible), and divide the miles on the trip by the litres added. Multiply litres by 4.54609 to convert to gallons (UK gallons, not US), then divide miles by gallons. Do this over three tanks to average out the variability in pump cut-off points.

Petrol vs diesel vs hybrid — which actually saves money?

The official MPG figures make diesels look like an obvious choice for high-mileage drivers, and for a long time they were. But with ULEZ expansion, the diesel tax differential narrowing, and modern petrol engines becoming genuinely efficient, the crossover point where diesel makes sense has moved a long way — you now typically need to do 15,000+ miles a year for the maths to work in diesel's favour. Full hybrids like the Toyota Prius and Yaris Hybrid genuinely deliver near-brochure MPG in urban use because the regenerative braking captures energy that would otherwise be wasted as heat. Plug-in hybrids only beat a petrol if you actually plug them in — the published figures assume you do, and owners who treat them as combustion cars routinely see figures worse than an equivalent petrol.

Using MPG to budget running costs

Once you know your real-world MPG, calculating annual fuel spend is trivial: divide your annual mileage by your MPG, multiply by the current fuel price per gallon (4.54609 × price per litre). A 40 MPG car doing 10,000 miles a year at £1.45/l petrol costs about £1,647 a year in fuel — or £137 a month. Our running cost calculator does this automatically, using live gov.uk fuel prices to give you an up-to-date figure. It's the single biggest variable cost of car ownership and worth sanity-checking before you buy.