What is MPG?

14 November 2019

The fuel efficiency of petrol and diesel vehicles is measured in Miles Per Gallon or MPG. This number represents the amount of miles that a car can potentially cover by burning one gallon of fuel (in the UK a gallon is approximately 4.5 litres). So if a car has a stated MPG of 50 that means burning a gallon of fuel you can theoretically cover 50 miles, although there are a number of other both internal and external factors which can affect performance of vehicle once it's out on the road.

Essentially you can think of MPG as the overall efficiency of a vehicle, it is a measure of distance driven using a specific amount of fuel. So even if you don't think of gallons normally when visiting the petrol station you can use the figure to compare options when looking for your next vehicle. Higher MPG figures mean using less fuel and a vehicle that will be cheaper to run over time.

Why do cars have different MPG figures?

A car's MPG depends on many different factors, for example

• Engine efficiency
  The amount of energy lost from friction between the mechanical parts of a vehicle will reduce its MPG. Energy can be lost to heat or sound for example rather than used to power the vehicle forwards.
• Regenerative braking
  A system whereby energy lost from braking is stored and then used to accelerate the vehicle can increase MPG by wasting less power.
• Car weight
  Generally speaking, it will usually require more energy to move heavier vehicles so the MPG achieved will be lower. Choosing a lighter vehicle could mean saving money when it comes to purchasing fuel.
• Engine performance
  It's not as simple to say that larger engines will burn less fuel, but often if an engine is more powerful it may well have a lower MPG and be more wasteful with fuel. Opting for a vehicle with a smaller engine could potentially translate to using less fuel to cover the same journeys.
• Aero-dynamics
  Less air resistance on a vehicle will mean a lower force of drag which will increase MPG and performance. This effect is especially noticeable at higher speeds, which is why sports cars tend to be designed with aero-dynamics very much in mind.
• Four wheel drive
  Four wheel drive will generally lower overall efficiency because the engine is having to power all of the wheels rather than just two, this can therefore translate to lower a MPG score.
• Petrol/Diesel
  At this moment in time diesel vehicles tend to report slightly higher MPG figures than petrol cars, although this could change, and also petrol hybrid cars can be some of the most efficient on the road. If you want to save money too, it's worth remembering to consider that the price of both fuels can fluctuate as well.
• Vehicle features
  All systems within a vehicle can potentially affect its fuel efficiency. Anything from power steering and suspension to air conditioning and window defrosters can reduce MPG as energy is required to drive them

Which external factors affect a vehicle's MPG?

Aside from a vehicle's efficiency there are a number of other factors which can have an impact on the MPG it will achieve once driven in the real world. Here are some of the main things that will affect fuel economy on the road: 

• Driving style
  Stopping and starting plus braking and accelerating more often than necessary will usually reduce fuel efficiency and MPG figures.
• Road and tyre conditions
  Driving on uneven surfaces or with low tyre pressure can reduce the efficiency of a vehicle and its MPG.
• Length of journey
  Using a vehicle for frequent short trips rather than longer journeys will generally reduce the efficiency of a vehicle and its MPG over time.
• Traffic
  Driving in urban areas with more traffic and signals can result in extra stopping and starting which will lower efficiency and MPG.
• Weather
  Poor weather conditions and lower temperature can reduce the performance of tyres and the efficiency of engines which could in turn lower the MPG

How is MPG tested?

In the past a system called NEDC (New European Driving Cycle) was used to evaluate the performance of vehicles based on how they might theoretically be driven, although often the figures claimed varied widely from on-road results. Since September 2018 all new vehicles have been graded using standardised testing known as WLTP, which stands for Worldwide Harmonised Light Vehicle Test Procedure. Whilst still far from perfect, these tests measure vehicles' MPG, emissions and alternative energy consumption in laboratory conditions using more realistic real world driving data and aim to provide more accurate figures than the previous regime.

How can I find out my car's MPG

To easiest way to test the MPG of your vehicle is to use its dashboard computer, which should tell you a figure for your recent driving. If this feature is not available you could perform a rough test by filling the tank and driving whilst recording the miles covered. When you come to refill your car you will be able to find out how many litres (and therefore gallons) of fuel you have used to drive the miles you have covered according to your odometer, which is your real life MPG for these journeys.

What are the different kinds of MPG?

Urban MPG

This is a term used to describe the fuel efficiency of vehicle when driven in a city, taking in to account the stopping, starting and relatively low speeds that this entails. Generally speaking, vehicles will not be able to drive as efficiently in these conditions so urban MPG will tend to be lower.

Extra Urban MPG

Fuel economy whilst driving outside of cities, on country roads or the motorway for example, is called extra urban MPG. This will mean more constant speeds with less braking and accelerating so more efficiency, which should translate in to a higher MPG figure.

Combined MPG

Most people will drive both inside and outside of cities and that's what combined MPG aims to put a figure on. Of course this will depend on each driver and their personal requirements, which can also change over time. Calculating a combined MPG can be useful to give an idea of how much fuel you might actually use in real-life taking in to account the different types of journeys that you will make in your vehicle. 

Do electric cars have MPG?

As electric cars don't use petrol or diesel they don't have an MPG figure as such, because the energy they use is measured in kWh (kilowatt hours) rather than gallons or litres. In order to compare the efficiency of an electric vehicle you can use something called MPGe which stands for Miles Per Gallon Equivalent.

How can you measure the efficiency of an electric car?

Electric vehicles tend to be much more efficient than their internal combustion engine cousins, but of course this is only half the story as there are other factors to take in to account in creating the electricity in the first place as well as extracting and transporting the combustible fuel.

Therefore, it only really makes sense to compare like for like, so when researching your next electric vehicle, you can use the size of the battery and range of the vehicle to give you an idea of how efficient each machine is. For example, comparing the Nissan Leaf with the Audi E-Tron you can see that the Audi has more than double the battery size but only around 30% more range and so the Nissan is more energy efficient. Given that the Audi is much larger and more powerful this of course is to be expected.

Vehicle	Battery	Range
Nissan Leaf 110kW N-Connecta 40kWh 5dr Auto	40kWH	235
Audi E-Tron Estate 300kW 55 Quattro 95kWh 5dr Auto	95kWH	310

What should I do if I think my MPG is wrong?

As explained earlier it's common to find that MPG in real life may not reflect the manufacturer's claim. However, luckily there are a number of things you can do to improve your fuel economy and therefore achieve better MPG figures for your vehicle:

• Try to brake less
  Obviously if you don't want to be in an accident it's important to use your brakes once in awhile. Trying to stay within speed limits and leave a larger stopping distance will not only make you a safer driver though, it will also save you money too, by increasing your fuel economy and helping your get more miles per gallon of fuel.
• Change up and often
  Moving up through the gears as early as possible is important to ensure your vehicle is operating at its most efficient. Although of course it's also important to remember to change down too so as to avoid putting undue strain on the engine.
• Reduce drag
  You might not be able to change the shape of your vehicle to make it more efficient, but try to keep with the windows and sun roof closed to improve fuel economy. This is especially important when at driving higher speeds such as during motorway driving.
• Avoid extra weight
  You may have noticed experiencing lower MPG figures when your car is full of passengers. Of course this is usually unavoidable, but many motorists find that they are driving around with things they don't need which add additional weight to every journey. Taking a look throughout your vehicle and removing anything you don't need could improve your vehicle's efficiency.
• Look after your tyres
  Ensuring that your tyres are pumped up to correct manufacturer-recommended pressure, and the treads are in tact, is an easy way to improve fuel economy and can have a dramatic effect on your fuel economy over time.
• Be sure to service
  To ensure that your vehicle is performing at its best, regular servicing is crucial. Something as simple as a clogged air filter can have a seriously adverse effect on your vehicle's efficiency.
• Don't be idle
  Whenever you're standing stationary with the motor running you're covering no miles but still burning fuel. Sometimes this is unavoidable but it's good practice to try and keep your engine off when it's not required. 
• Use your gadgets wisely
  It's worth bearing in mind that everything in your vehicle from the heaters to air conditioning or the radio and computer system can use energy. However for most drivers using cruise control can lead to a smoother ride and increase efficiency.