What is fuel economy?

Why fuel economy matters

Fuel economy is one of the main numbers behind journey cost. A car that travels further on the same amount of fuel will usually cost less per mile than a less efficient car, assuming fuel prices are the same.

It also affects how often you need to refuel or recharge, how sensitive your budget is to price changes, and whether a petrol, diesel or electric vehicle is cheaper for your normal use.

Fuel economy vs MPG

MPG is one way to express fuel economy. It tells you how many miles a petrol or diesel car can travel on one UK gallon of fuel.

Fuel economy is the broader idea. It can include petrol, diesel, hybrid and electric efficiency. For EVs, a more useful measure is often miles per kWh, because electricity is measured in kilowatt-hours rather than gallons.

How fuel economy is measured

For petrol and diesel cars, the basic calculation uses miles travelled and fuel used. If you know the number of litres used, convert litres into UK gallons first.

For an electric vehicle, divide the distance by the electricity used. Charging losses can make the wall-to-wheel cost slightly higher than the energy added to the battery.

Worked example

Suppose you drive 300 miles and use 35 litres of petrol.

You can then use that MPG figure in the fuel cost calculator to estimate the cost of future trips.

What affects fuel economy?

Real-world fuel economy can differ from official figures. A car may use more energy in traffic, cold weather, short trips or stop-start town driving.

• Driving style: harsh acceleration and late braking usually reduce efficiency.
• Speed: motorway speeds can increase drag and use more fuel or electricity.
• Tyres: low tyre pressure can increase rolling resistance.
• Vehicle load: extra weight, roof boxes and trailers can reduce efficiency.
• Weather: cold weather can reduce EV range and increase fuel use.
• Route type: steady routes usually produce better results than congested journeys.

Fuel economy for petrol, diesel and EVs

Petrol and diesel comparisons normally start with MPG, then convert the result into cost using the fuel price per litre. EV comparisons usually start with miles per kWh, then use the electricity tariff.

An EV tariff can make a big difference because charging overnight at home may be cheaper than using a public rapid charger. That is why a single efficiency figure does not tell the whole cost story.

How to improve fuel economy

You cannot change a vehicle’s design, but you can often improve real-world efficiency with small changes.

• Keep tyres inflated to the recommended pressure.
• Remove unnecessary weight from the boot.
• Drive smoothly and look further ahead in traffic.
• Use air conditioning sensibly rather than leaving it on by habit.
• Plan routes that avoid repeated stop-start traffic where possible.
• For EVs, pre-condition the cabin while plugged in if your vehicle supports it.

Useful calculators for fuel economy

Use these tools depending on what you are trying to work out.

Common mistakes

• Mixing US and UK gallons: UK MPG uses imperial gallons, not US gallons.
• Comparing MPG with miles per kWh: they measure different energy units.
• Ignoring fuel price: good efficiency can still cost more if the energy price is high.
• Using official figures as guarantees: real-world results depend on use, route and conditions.
• Only checking fuel: a cheap-to-fuel car can still be expensive if other ownership costs are high.

Fuel economy FAQs