What is MPG? A Complete Guide to Fuel Economy Figures

What Does MPG Mean?

MPG stands for Miles Per Gallon and is a unit of measure for a car’s fuel economy. Simply put, it tells you how many miles a car can travel on one gallon of fuel. For instance, if a car is rated at 45.6mpg, it can go 45.6 miles on a single gallon of fuel. At Cazoo, we use the ‘official’ average MPG figures published by a car’s manufacturer, ensuring accurate and reliable data.

How is MPG Measured?

The procedures for measuring a car's fuel economy have evolved over the years. The current standard is the Worldwide Harmonised Light-Duty Vehicle Testing Procedure (WLTP). This test is conducted in a lab, but is designed to reflect real-world driving conditions. Cars are driven on a rolling road through a series of acceleration runs, braking events, and cruising at different speeds. The amount of fuel used is measured, and a simple calculation reveals the car's MPG. Alongside WLTP, cars are also put through a separate on-road Real Driving Emissions (RDE) test, which uses portable equipment to check emissions during everyday driving rather than just in the lab.

What’s the difference between NEDC and WLTP?

Before WLTP, the New European Driving Cycle (NEDC) was used across Europe. While it created a level playing field for all cars, most owners found their vehicles didn't match the 'official' MPG. The WLTP provides lower and more realistic numbers, making it a better measure of real-world fuel economy. WLTP became mandatory for approving new car models from September 2017, and for every new car sold from September 2018, so if your car was registered from late 2018 onwards, its figures will have come from a WLTP test rather than the older NEDC one.

Why are there several different MPG figures for every car?

Manufacturers publish several different MPG figures because a car uses fuel very differently at low speeds around town compared with cruising on the motorway. Under the current WLTP test, each car is given a figure for five separate phases:

• Low: typical of slow-moving, stop-start city driving.
• Medium: general town and suburban driving.
• High: faster A-roads and rural routes.
• Extra High: motorway speeds, up to around 81mph.
• Combined: an overall average across all four phases.

This replaced the older NEDC system, which only gave Urban, Extra-urban and Combined figures – so if you're reading an older road test or brochure, you may still see those terms used instead.

For most drivers, the Combined MPG figure is the most relevant, as it closely matches everyday driving conditions.

How accurate are official MPG figures?

While official MPG numbers offer a good guide, your real-world fuel economy can vary based on your driving habits, the weather, how heavily loaded the car is, and the roads you use most. WLTP figures are far closer to reality than the old NEDC ones ever were, but there's still a gap: independent MPG testing has found that official WLTP figures are, on average, around 5% higher than what cars actually achieve on the road. That's a huge improvement on NEDC, where the gap was often 20% or more, but it's still worth treating the official number as a best-case guide rather than a guarantee.

Plug-in hybrid cars are the biggest exception to this. Their official MPG figures often look extremely high, but that's because the test assumes the battery starts fully charged and is used for a large part of the test. In the real world, a plug-in hybrid's fuel economy depends heavily on keeping the battery topped up and driving the sort of shorter journeys the electric motor can cover – run one on a long motorway trip with a flat battery and the MPG will be much closer to that of a regular petrol car.

How do I calculate my car’s MPG?

Want to know exactly what MPG your car delivers? Here’s how you can calculate it:

1. Fill your car’s fuel tank until the pump clicks off. Note the mileage on the odometer or reset the trip computer.
2. Next time you fill up, note the amount of fuel used (in litres) and the mileage.
3. Divide the fuel amount by 4.546 to convert litres to gallons.
4. Divide the mileage by the gallons used.

For example:

52.8 litres ÷ 4.546 = 11.615 gallons

368 miles ÷ 11.615 gallons = 31.683mpg

What does L/100KM mean?

L/100KM stands for Litres per 100 Kilometres, another unit of measure for fuel economy used in Europe and other metric system countries. To convert L/100KM to MPG, divide 282.5 by the L/100KM figure.

Can I improve my car’s MPG?

Improving your car’s MPG can save you money and reduce your carbon footprint. Here are some tips:

1. Aerodynamics: Keep windows closed and remove roof racks when not in use to improve airflow.
2. Tyre Pressure: Maintain correct tyre pressure to reduce friction and improve fuel efficiency.
3. Reduce Weight: Remove unnecessary items from your car to lighten the load.
4. Electrical Equipment: Use air conditioning and other electrical equipment sparingly.
5. Regular Servicing: A well-maintained engine performs better and is more fuel-efficient.

Does how I drive affect my car’s MPG?

Absolutely! Your driving style can significantly impact fuel economy:

• Smooth Acceleration and Braking: Avoid rapid acceleration and hard braking.
• Optimal Gear Use: Shift gears appropriately in manual cars to keep the engine running efficiently.
• Eco Mode: Use your car's eco mode if available to optimise fuel efficiency.

Our guide to driving more economically goes into more detail on all of this, with plenty of extra tips to help you get the most from every tank.

What cars give the best MPG?

Generally, smaller cars offer better fuel economy. However, larger diesel and hybrid vehicles can also be efficient, often achieving 60mpg or more, and some full hybrids and plug-in hybrids post official combined figures well beyond that. If you're looking for cars with excellent fuel economy, check out our most fuel-efficient cars guide, or our roundup of cheap fuel-efficient used cars if budget is your main priority.