Real MPG: why your car never matches the brochure

Almost nobody gets the fuel economy printed in the brochure. That is not because the manufacturer lied, it is because the figure was never a promise about your driving.

Where the number comes from

Official economy figures come from a standardised laboratory test. Since 2017 that has been WLTP, which replaced the older and much more flattering NEDC cycle. WLTP is a great deal closer to reality than what came before, but it is still a controlled procedure: fixed temperature, a defined speed profile, no headwind, no roof box, no traffic, no you.

Its job is to let you compare two cars fairly. It was never designed to predict what your commute costs.

Why your number is lower

• Short trips. A cold engine is a thirsty engine. Two miles to the station can return half what the same car does on a motorway run.
• Speed. Air resistance climbs with the square of speed. Seventy costs a lot more than sixty.
• Weight and drag. Roof bars, a boot full of tools, a bike rack you left on since June.
• Winter. Denser air, warm-up cycles, lights and heaters. A 10% seasonal swing is unremarkable.
• How you drive. Anticipation is worth more than any gadget you can buy.

Working out the number that actually matters

You only need two things: a full tank, and the next full tank.

1. Fill the tank completely and note the odometer.
2. Drive as you normally would.
3. Fill it completely again, note the odometer and the litres that went in.

Then:

MPG = (miles driven ÷ litres) × 4.546

That 4.546 is the litres in an imperial gallon, which is what UK mpg is quoted in. Miss it and you will produce a US mpg figure that is 20% too low, which is a classic way to convince yourself something is wrong with the car.

One tank is a snapshot. Three or four gives you the number to plan with.

What it is worth knowing

Real economy is what turns a distance into a cost. At 145.9p a litre, a 40-mile round trip costs about £5.80 in a car doing 52 mpg, and about £8.60 in the same journey at 35 mpg. Over a working year that difference is a holiday.

It also decides whether the 55p a mile you can claim is generous or barely covers the fuel. For an efficient car it comfortably covers running costs. For a thirsty one towing a trailer, it is doing well to break even.

Milesheet works this out for you: log each fill-up with the litres, the cost and the odometer, and it derives your real MPG between full tanks, then prices every journey with your own number instead of the brochure's.

What the gap typically looks like

Real-world economy usually lands below the official figure. The size of the gap varies by powertrain more than by brand:

The plug-in case is the one that misleads most. A PHEV driven without charging returns something close to a heavy petrol car, not the headline figure.

Working out your own, properly

You need two full-tank fills with odometer readings:

The brim matters more than the arithmetic. Filling to the first click both times is what makes the two measurements comparable; a splash-and-dash in the middle is fine as long as you add its litres in.

What it changes

A 25% error in economy is a 25% error in every journey cost you calculate, including what you tell friends they owe for petrol.

Sources

• Travel, mileage and fuel rates and allowances (HMRC)
• Simplified expenses checker (HMRC)
• Vehicle tax rate tables (DVLA)