How to Check Your Car's Real MPG — Brim-to-Brim Test & Tips

The MPG figure on your car's spec sheet is measured in a lab under ideal conditions. Real-world fuel economy is almost always worse. Here is how to measure what your car actually achieves and what you can do to improve it.

Why Official MPG Figures Are Wrong

Since September 2017, new cars sold in the UK are tested using the WLTP (Worldwide Harmonised Light Vehicle Test Procedure), which replaced the older NEDC cycle. WLTP is more realistic than NEDC, but it is still a laboratory test with controlled temperatures, no wind, no passengers, and no real-world traffic conditions.

According to data from Emissions Analytics, the average real-world MPG shortfall is around 15 to 20 percent compared to the official WLTP combined figure. For some cars, particularly hybrids and turbocharged petrol engines, the gap can be 25 percent or more.

Method 1: The Brim-to-Brim Test

This is the most accurate way to measure your real fuel economy. It requires two fill-ups and a trip meter or odometer reading.

1. Fill your tank completely at a fuel station. Stop when the pump clicks off automatically. Note your trip meter reading (or reset it to zero).
2. Drive normally for at least 200 miles. The longer the distance, the more accurate the result. Mix in your typical driving — commuting, motorway, town.
3. Return to a fuel station and fill up completely again, stopping when the pump clicks off. Note the litres pumped and your trip meter distance.
4. Calculate your MPG: divide the miles driven by the litres used, then multiply by 4.546 to convert to miles per imperial gallon.

For a quick calculation, use our MPG calculator — just enter the distance and litres.

Method 2: Your Car's Trip Computer

Most modern cars have a trip computer that displays average fuel consumption. This is convenient but not always accurate. Trip computers typically overestimate fuel economy by 5 to 10 percent because they calculate from injector pulse data rather than measuring fuel flow directly.

Use the trip computer as a rough guide, but verify with a brim-to-brim test at least once to understand the offset for your specific car.

What Affects Your Real-World MPG?

• Driving style — aggressive acceleration and hard braking can reduce MPG by 15 to 30 percent compared to smooth, anticipatory driving.
• Speed — fuel consumption increases significantly above 60 mph. Driving at 80 mph uses roughly 25 percent more fuel than 60 mph.
• Tyre pressure — under-inflated tyres increase rolling resistance. Tyres that are 10 psi below the recommended pressure can reduce MPG by 3 to 5 percent.
• Weight — every 50 kg of extra weight costs roughly 1 to 2 percent in fuel economy. Clear out the boot regularly.
• Air conditioning — running the A/C can reduce MPG by 5 to 10 percent, particularly in city driving. At motorway speeds, the effect is smaller.
• Cold weather — engines are less efficient when cold, and winter fuel blends have slightly less energy content. MPG can drop 10 to 15 percent in winter.
• Route type — motorway cruising is more efficient than stop-start urban driving. A commute through city traffic can halve the MPG you would achieve on a long motorway run.

How to Improve Your MPG

1. Check tyre pressures monthly — this is the easiest and cheapest improvement. Most petrol stations have free air pumps.
2. Accelerate gently — aim to reach your target speed smoothly rather than flooring it. In a manual, shift up by 2,500 rpm for petrol or 2,000 rpm for diesel.
3. Read the road ahead — anticipate traffic lights, roundabouts and junctions to avoid unnecessary braking and acceleration.
4. Use cruise control on motorways — maintaining a steady speed reduces fuel waste from speed fluctuations.
5. Remove roof boxes and racks — even an empty roof rack adds aerodynamic drag that costs 1 to 2 MPG at motorway speeds.
6. Keep windows closed at speed — open windows at 60+ mph create significant drag. Use the ventilation system instead.
7. Plan your route — avoid congested roads when possible. A longer route with flowing traffic can use less fuel than a shorter route through town.

For more tips, read our complete guide to saving money on fuel. To see how much your improved MPG could save you per year, try the fuel savings calculator.