Why Real-World MPG Is Lower Than the Official Figure

Official mpg is measured in a laboratory test so that cars can be compared fairly, not to predict your own fuel use. Your figure will differ because of speed, load, short trips, cold starts and tyres. Speed is the one you can quantify: a petrol van uses 16.1% less fuel per mile at 60 mph than at 70 mph (DfT).

What is the official mpg figure based on?

It is based on WLTP, the Worldwide Harmonised Light Vehicle Test Procedure, which is carried out in a laboratory. The Vehicle Certification Agency (VCA) says that under WLTP “all tests will continue to be conducted in a laboratory allowing meaningful comparisons between cars to be made”. Its page on the WLTP says WLTP replaced the older NEDC test, and that results are reported as Low, Medium, High, Extra-high and Combined.

The point of the test is comparison. Two cars run the same procedure, in the same type of conditions, so a buyer can see which uses less. It is not a forecast of what any one driver will get.

Why is real-world mpg lower than the official figure?

Because no road, driver or journey matches the test exactly. The VCA’s fuel consumption FAQ puts it plainly: “Because of the need to maintain strict comparability of results achieved by the standard tests they cannot be fully representative of real-life driving conditions. There are infinite variations in driving styles and in road, car and weather conditions.”

The VCA also says WLTP data is “a closer representation” of what you are likely to achieve in the real world than the old NEDC figures were. Closer is not the same as equal. We have not found an official figure for how much lower real-world mpg is on average, so we do not quote one. What the sources do support is a list of the reasons.

What makes real-world mpg lower?

Several things, and they stack up. None of the following has a single published percentage that fits every vehicle, so treat the list as a guide to where to look.

Cause	What happens
Higher speed	Air resistance rises sharply with speed, so each mile uses more fuel the faster you go.
Heavy load	More weight needs more energy to accelerate and to climb hills. A van full of stock or tools uses more than an empty one.
Cold starts and short trips	A cold engine and cold oil use more fuel until they warm up. A vehicle that only does short hops never fully warms.
Stop-start driving	Repeated braking and accelerating wastes energy. Congestion and delivery rounds are harder on fuel than steady running.
Tyres	Under-inflated or worn tyres drag more. Check pressures against the vehicle handbook.
Driving style	Hard acceleration and late braking burn fuel that is lost as heat.
Weather and equipment	Cold weather, heating, air conditioning, roof racks and open windows at speed all add to fuel use.
Idling	A running engine at a standstill does zero miles to the gallon.

A fleet vehicle often sits at the hard end of several of these at once: loaded, stopping and starting, and driven to a schedule.

How much does speed change fuel use?

More than any other cause on the list, and this is the only one with published figures. The Department for Transport’s fuel consumption curves in its TAG data book (v2.03, May 2026, table A1.3.8) give fuel per mile at each average speed. This is how much less fuel per mile you use at the lower speed:

Vehicle	70 to 60 mph	68 to 60 mph	68 to 56 mph
Diesel van	not covered	14.5%	20.0%
Petrol van	16.1%	12.6%	16.4%
Diesel car	9.6%	7.5%	10.1%
Petrol car	8.9%	6.8%	8.8%

DfT’s diesel van curve stops at 68 mph, so we do not quote a diesel van figure above that. These are modelled curves of fuel against average speed, not a test of any one vehicle.

The lesson for a fleet: the faster your drivers run on fast roads, the larger the gap is likely to be. That is a decision you can control. For the full ranking of what helps, see which driving technique can save fuel.

Why is a van or fleet vehicle often further from the brochure?

Because a working vehicle carries loads, makes frequent stops and runs to deadlines. A brochure number is for a car under test conditions. A van with racking, tools and parcels, doing drops in town and motorway runs between depots, is a very different duty.

That is why published figures are the wrong basis for budgeting. A cost sum built on the brochure mpg will understate your bill. A sum built on your own measured mpg will not.

What should a fleet do instead of trusting the official figure?

Measure your own mpg for each vehicle, over a few hundred miles at least, and budget on that. Our guide on how to calculate mpg shows the brim-to-brim method and how to use fuel card data.

Then track it over time. A vehicle whose mpg falls against its own history points to a tyre fault, a mechanical issue, a heavier load or faster driving. The fuel cost calculator turns your measured mpg into a cost per mile and an annual bill at this week’s DESNZ prices.

Frequently Asked Questions

Why is my mpg lower than advertised?

Advertised mpg comes from a laboratory test designed for fair comparison between cars, not to match any one driver’s roads, load or speed. The Vehicle Certification Agency says the standard tests cannot be fully representative of real-life driving. Higher speeds, loads, short trips and cold starts all lower your figure.

What is WLTP mpg?

It is the official fuel economy figure for a new car, measured using the Worldwide Harmonised Light Vehicle Test Procedure in a laboratory. It replaced the older NEDC figures, and the VCA describes it as a closer representation of real-world results than NEDC gave. It is still a lab figure.

Is real-world mpg always lower than the official figure?

For most drivers it is lower, but we do not quote a percentage because we have not found an official source for one. Your gap depends on speed, load, journey type and driving style. Measure your own figure and use that.

Which factor has the biggest effect on mpg?

Of the causes we can put a number on, speed. The Department for Transport’s curves show a petrol van using 16.1% less fuel per mile at 60 mph than at 70 mph, and a petrol car 8.9% less. Other causes matter, but have no single published figure.

Can a speed limiter close the gap?

It can narrow it where the vehicle would otherwise run faster than the cap. The size of the saving depends on how many of your miles are driven above the cap. A limiter does nothing for the miles driven below it.

Cut your fleet’s fuel bill

Find your own number with the fuel cost calculator. Enter your measured mpg rather than the brochure figure, and it shows the cost per mile and the annual bill using this week’s prices.

A speed cap saves fuel on every journey without the driver having to remember it. The Department for Transport’s fuel consumption curves show a diesel van using 14.5% less fuel per mile at 60 mph than at 68 mph. How much of that reaches your bill depends on how many of your miles are driven above the cap. See fuel savings from speed limiters.

Talk to us. AutoKontrol has made speed limiters in Britain since 1985, with nationwide installation where we come to you. Book a free consultation or get a quote.