Fuel Consumption Explained: What It Means for UK Drivers

Fuel consumption is simply how much fuel your car uses to cover a given distance. In the UK it is normally quoted as miles per gallon, where a higher number is better, or as litres per 100km, where a lower number is better. Both describe the same thing from opposite directions, and the figure your car actually achieves depends on three things: the vehicle, the journey and the driver. Only one of those is fixed.

What is fuel consumption and how is it measured?

Fuel consumption is the volume of fuel used per unit of distance travelled. The UK sits awkwardly between two measurement systems, which is why you see both units in use.

Two conversions worth knowing. To convert between the units, divide 282.5 by the figure you have: 40mpg is about 7.1 L/100km, and 6 L/100km is about 47mpg. And note that the UK uses the imperial gallon of 4.546 litres, which is larger than the US gallon, so a US mpg figure looks worse than the equivalent UK one for the same car. Any American review quoting mpg is using the smaller gallon.

How are official fuel figures produced?

Official economy figures come from a standardised laboratory test called WLTP, which replaced the older and much more optimistic NEDC test. WLTP uses a longer, more varied drive cycle with higher speeds and harder acceleration, and it accounts for optional equipment fitted to the specific car, so the figures are closer to reality than the ones they replaced.

They are still laboratory figures. The test runs at a controlled temperature with a warm car, no headwind, no hills, no traffic, no passengers and no roof bars. That is deliberate, because the point of the test is to let you compare one car against another on the same basis rather than to predict your commute. Expect your own figures to fall short of the quoted combined number, and expect the gap to be widest on short urban journeys.

What affects how much fuel a car uses?

How do you calculate your own mpg?

Trip computers are useful for trends but they are estimates, and most read a few per cent optimistic. To get a true figure, use the brim to brim method.

1. Fill the tank until the pump clicks off, then stop. Do not keep squeezing.
2. Reset the trip meter to zero.
3. Drive normally until the tank is somewhere around a quarter full.
4. Return to the same pump at the same station and fill to the first click again.
5. Note the litres on the receipt and the miles on the trip meter.
6. Divide miles by litres, then multiply by 4.546. That is your mpg.

As a worked example, 320 miles on 34 litres gives 320 divided by 34, which is 9.41, multiplied by 4.546, which is about 42.8mpg.

Repeat across two or three tanks and average them. A single tank dominated by one long motorway run or one week of school runs will not represent your normal driving.

Which uses less fuel: petrol, diesel, hybrid or electric?

Diesel engines are more thermally efficient than petrol ones and diesel fuel contains more energy per litre, so a diesel car generally returns better mpg than an equivalent petrol, particularly on long motorway journeys. That advantage narrows or disappears in town, where a diesel particulate filter needs regular longer runs to regenerate properly and short trips cause problems.

The old advice that diesel is automatically the economical choice no longer holds, because it depends on how many miles you do and where. A driver covering 6,000 urban miles a year will rarely recover the extra purchase cost of a diesel, and may face charges in clean air zones with an older one.

How can you cut fuel use without changing car?

Driving technique is the biggest single lever available to most drivers, and it costs nothing.

• Accelerate gently and read the road ahead. Every time you brake, you throw away fuel you already burnt getting up to speed.
• Change up early in a manual, keeping revs low without labouring the engine. In an automatic, easing off the throttle prompts an earlier upshift.
• Hold a steady speed. Cruise control helps on flat motorways, though on hilly roads it can hold speed too aggressively and use more.
• Ease off the motorway speed. Drag rises steeply with speed, so the difference between 70 and 80mph is far larger than the difference between 50 and 60.
• Do not idle unnecessarily. Idling burns fuel and covers no distance, and leaving an engine running while stationary on a public road can attract a fixed penalty from the local authority.
• Combine journeys. One longer trip with a warm engine uses less fuel than three short cold ones covering the same distance.
• Take the weight out. Empty the boot of what you do not need and remove roof bars and boxes when they are not in use.
• Check tyre pressures monthly. Underinflation is one of the most common and most easily fixed causes of poor economy.
• Keep up with servicing. Clean filters, correct grade oil and healthy plugs all matter.
• Plan the route. Avoiding a known jam saves more fuel than any driving technique, and cold, short, congested trips are where economy is worst.

Which fuel saving claims are worth ignoring?

Plenty of advice passed around about fuel economy either does nothing or costs you money.

• Bolt on fuel savers. Magnets clamped to the fuel line, in tank pills and plug in devices that claim a large percentage gain have no credible evidence behind them. If a manufacturer could add a cheap component and gain ten per cent, it would already be fitted.
• Premium fuel in an ordinary car. Higher octane petrol benefits engines designed to use it. In a car built for standard unleaded, you are usually paying more for no measurable difference.
• Filling up in the cold morning air. The theory is that denser fuel gives you more per litre. Underground tanks sit at a fairly stable temperature, so the effect is far too small to notice.
• Coasting in neutral. Modern engines cut fuel entirely on the overrun when you lift off in gear. Rolling in neutral actually uses fuel to keep the engine idling, and it removes engine braking, so it is slower and less safe.
• Running the tank almost empty to save weight. The weight saved is trivial next to the risk of running the fuel pump dry or picking up sediment.
• Switching the engine off at every stop. Manufacturer stop start systems are designed for it, with a suitable starter and battery. Doing it manually in a car without one adds wear for very little gain.

Engine remapping is the one modification that genuinely changes consumption, but it must be done by a reputable specialist, it may affect your warranty, and it has to be declared to your insurer or your cover can be invalidated.

Does bodywork condition affect fuel consumption?

It can, and it is the part most drivers overlook. Fuel economy at speed is largely a fight against aerodynamic drag, and modern cars are shaped and panelled to manage airflow closely.

• A missing or damaged front spoiler, splitter or underbody tray disturbs airflow beneath the car. These plastic trays are cheap, easily torn off by kerbs and speed bumps, and often not replaced.
• A poorly fitted or damaged bumper leaves gaps and steps in the airflow, and a cracked one can flex at speed.
• Missing wheel arch liners let air spill into the arches and also allow salt and water into the panel behind, which is a corrosion problem as well as a drag one.
• Doors, bonnets or boot lids that no longer sit flush after a knock create turbulence and wind noise, and often leak water too.
• Roof bars, aftermarket light bars and oversized spoilers all add drag permanently.

None of these will double your fuel bill, but they add up alongside the more obvious reasons to put them right. If a car has taken a knock, having the bodywork repaired properly restores the panel gaps and the underbody protection rather than leaving a car that is untidy, noisy and slightly less efficient. Damaged or ill fitting bumpers are worth attention for the same reason, since they carry a lot of the front and rear aerodynamic shaping.

What UK rules and costs relate to fuel use?

Several parts of UK motoring cost are tied to how much fuel a car uses and what it emits.

• Fuel duty and VAT make up a large share of the pump price, so fuel efficiency directly changes your tax burden as well as your fuel bill.
• Vehicle Excise Duty, the road tax collected by the DVLA, is based on CO2 emissions for the first year on new cars and then moves to a standard rate, with an additional supplement for cars above a set list price. Rates change at Budgets, so check the current bands on GOV.UK rather than relying on an old figure.
• Clean air zones and low emission zones charge older, higher emission vehicles to enter certain city centres. Schemes differ by city and the rules in Greater Manchester have been revised more than once, so check with the relevant local authority before travelling.
• Company car benefit in kind is calculated from CO2 emissions, which is why low emission and electric cars are so attractive as company vehicles.
• MOT emissions testing checks that the car meets the limits for its type, and a failed emissions test usually points at something that is also costing you fuel.

The direction of travel

New petrol and diesel car sales in the UK are being phased out under government policy, with hybrid and electric taking their place, and the charging network continues to expand. For most drivers the practical question is not whether to switch immediately but what makes sense over the life of their next car: annual mileage, the mix of urban and motorway driving, whether home charging is possible, and whether the car will spend time in a charging zone.

Whatever you drive, the fundamentals do not change. A well maintained car with correct tyre pressures, no unnecessary weight, tidy bodywork and a driver who reads the road ahead will always use less fuel than the same car neglected.