How do I actually improve my car's fuel economy?
Which changes make a measurable difference to miles per gallon, which are folklore, and why your car's own display is probably flattering you.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
Short answer
The three things that genuinely move the figure are speed, smoothness and tyre pressure — steady driving with early gearchanges and correctly inflated tyres beats every additive and gadget on the market. Cold short journeys are the biggest single drain, because an engine uses far more fuel until it reaches temperature and a two-mile trip never gets there.
Fuel economy conversations are full of confident nonsense, partly because most of the classic advice was written for carburettor engines and partly because the effects are hard to see without measuring. Modern engines shut fuel off completely on the overrun, adjust mixture continuously, and stop at junctions. That changes which habits matter. It is worth knowing what is real before spending money on things that are not.
Step by step
- Measure properly before changing anything.Fill to the click, note the mileage, drive normally for a full tank, fill to the click again, and divide the miles by the litres used. Multiply by 4.546 to get miles per gallon. The trip computer is a useful trend indicator but is frequently optimistic by several per cent, so do not judge a change by it alone.
- Slow down on fast roads.Air resistance rises with the square of speed, so it is the dominant force above about 50 mph. Dropping from 80 to 70 mph on a motorway is the single biggest lever most drivers have and costs only a few minutes on a typical journey.
- Drive smoothly and read further ahead.Every time you brake you are throwing away fuel you already burned to build that speed. Looking further up the road so you can ease off rather than brake is where most of the difference between two drivers in the same car comes from.
- Change up earlier than feels natural.Modern engines make useful torque low down, and running at lower revs in a higher gear uses less fuel. Many cars have a gearchange indicator; it is worth trusting for a week to recalibrate your instinct.
- Keep the tyres at the right pressure.Underinflated tyres increase rolling resistance and cost fuel as well as wearing out faster. It is the cheapest single maintenance item with a measurable effect, and it needs checking monthly and after every big temperature change.
- Take the roof bars off when you are not using them.An empty roof rack or roof box is pure aerodynamic drag at exactly the speeds where drag dominates. This is one of the few bolt-on changes with a genuinely large effect.
- Lose the dead weight, but keep it in perspective.Emptying a boot full of things you are not using helps, particularly in town driving with a lot of acceleration. It matters much less than speed and smoothness on a long run.
- Combine short trips into one longer one.A cold engine can use dramatically more fuel in its first mile or two. Three separate two-mile trips are far more expensive than one six-mile trip that covers the same errands.
- Use the air conditioning sensibly rather than avoiding it.It does use fuel, mostly at low speed. At motorway speed, open windows create enough drag that the difference between windows down and air conditioning on is small either way. Do not sweat through a journey for a rounding error.
Common mistakes
- Buying a fuel-saving additive or device — Nothing sold as a bolt-on gadget has ever produced a reliable, repeatable saving in independent testing. The money is better spent on tyres and a service.
- Coasting in neutral to save fuel — It is a leftover from carburettor days and is now counterproductive as well as unsafe. A modern engine cuts fuel entirely when you lift off in gear; in neutral it has to idle, which uses fuel.
If it doesn't work
Economy suddenly got worse with no change in driving
Cause: A dragging brake, a fault triggering a rich mixture, a blocked air filter, or low tyre pressures — Fix: Check pressures first, then feel each wheel for heat after a drive. If nothing obvious, get the fault codes read — a failing sensor often causes a mixture change long before it lights a warning.
Winter economy much worse than summer
Cause: Cold engines, thicker oil, more time on heated screens and lights, and winter fuel blends — Fix: Normal and expected. A drop of ten per cent or more through the coldest months is common, and short journeys make it much worse.
Diesel economy poor and the car often runs a regeneration
Cause: Journey pattern too short for the particulate filter — Fix: Each regeneration burns extra fuel deliberately. If it happens often, the driving pattern rather than the car is the cause.
Trip computer says one thing, brim-to-brim says another
Cause: Trip computers are calculated estimates and are usually optimistic — Fix: Trust the brim-to-brim figure for absolute numbers and the computer for spotting trends.
Questions people ask
Does stop-start actually save anything?
Yes, in stop-start traffic, which is what it is for. On a car doing mostly open-road driving it barely engages. The batteries and starters fitted to those cars are built for the extra cycles, so switching it off routinely saves nothing and costs a little.
Is it cheaper to fill up more often or run the tank low?
The weight difference of a full tank is real but tiny compared with speed and driving style. Running consistently low is also hard on a fuel pump, which relies on the fuel to keep it cool. Fill when convenient.
Why does my car never get near its official figures?
Official figures come from a standardised laboratory cycle designed to be comparable between cars, not to predict your commute. The newer WLTP figures are closer to reality than the old NEDC ones, but a gap of ten to twenty per cent in real driving is entirely normal.