Everything you need to know about mpg

3rd January 2020

8 min read

You’ve probably heard of miles per gallon, or mpg as it’s more commonly known as. If you take a look at any car on our website, you’ll see it as part of the vehicle’s technical specification. But have you ever stopped to think what it actually means? Well, let’s find out.

What is miles per gallon (mpg)?

Miles per gallon is a measurement used to indicate how economical a car is. The higher your car’s mpg, the less fuel it uses, as you get more miles of driving from one gallon of fuel.

That means you have to spend less time filling up, resulting in more money in your pocket.

It’s worth noting that when the term gallon is used, it’s referring to an imperial or UK gallon, which equates to around 4.55 litres.

Why is mpg important?

Knowing a vehicle’s mpg, particularly if you’re thinking of leasing or buying that vehicle, is important as it’ll help you work out how much you can expect to spend on fuel.

It should be stated, however, that no manufacturer-released miles per gallon figure is ever 100% accurate, as everyone’s driving style is different. But it’s usually very close.

And despite it not being exact, when comparing manufacturers’ mpg, you’ll still be able to see which vehicle is more economical.

Miles per gallon figures also give you an idea of a vehicle’s CO2 emissions; the gas that exits through the exhaust in petrol, diesel and hybrid vehicles. Usually, the higher the mpg, meaning the more economical the engine, the lower the CO2 emissions – and vice-versa.

Although not directly related to a vehicle’s mpg, taking CO2 emissions into account is important when choosing a car. Not only do the emissions damage the Earth’s atmosphere, but they also define your road tax band. The higher the emissions, the greater your carbon footprint and the more road tax you’ll have to pay.

How do you calculate miles per gallon?

For every vehicle, you’ll be able to find its estimated miles per gallon. The manufacturer’s brochure or even a quick search on Google should do the trick.

Having a high mpg is a key selling point for several cars. But have you ever wondered how the number is calculated?

In a perfect world, it would be the result of real-road tests conducted by everyday drivers over a prolonged period of time. But as car manufacturers simply don’t have the time to complete such research before unveiling their new vehicles, the test is completed inside a lab.

To determine a vehicle’s mpg, there are two tests that manufacturers can carry out. The more common in today’s world is the Worldwide Harmonies Light Vehicle Test, or WLTP. Unlike the older method – which was called the new European driving cycle, or NEDC – the WLTP is based on real driving data.

By using the WLTP, the results are more likely to be closer to those seen out on the road in the real world, as the NEDC was only theoretical-based.

However, if you’re interested in your own vehicle’s miles per gallon and don’t have access to a state-of-the-art lab, it’s fairly easy to conduct your own research.

If it’s a modern vehicle, your car will most likely have a trip computer. In there, you’ll usually be able to find your average mpg.

However, if not, or you want to see how a particular route affects your fuel economy, such as your commute to work, you can do it manually.

Note down your vehicle’s mileage using the odometer when you next fill up. This is important to keep safe as you’ll need it as a reference point later.
Work out in gallons how much fuel you put in. You can do this by taking the number of litres and dividing by 4.55.
Drive until you need to fill up again then work out how many miles you’ve covered since your last fuel stop. To do this, subtract your first mileage reading (the one we said was important a moment ago) from your vehicle’s mileage on your second stop.
Divide the number of miles by the gallons of fuel you initially put into your car. Then, you’ll have your mpg.

Usually, the number you’re left with is your vehicle’s Combined mpg. And to understand what that means, we first have to look at two other types of mpg – Urban and Extra Urban.

Miles between fuel stops ÷ gallons of fuel used = mpg

What is Urban mpg?

Urban mpg is your vehicle’s miles per gallon figure in a typical city environment. The test to determine the figure, which is part of the Worldwide Harmonies Light Vehicle Test we explained earlier, is supposed to represent the start-stop motoring of modern-day city driving.

Over a distance of 2.5 miles, the Urban Cycle test consists of accelerating, decelerating, idling and steady speed testing at a maximum of 31mph. It’s designed to start with the vehicle having a cold engine to represent real life as closely as possible.

How accurate is Urban mpg?

As we’ve already said, a manufacturer-stated mpg is always pretty close even if rarely 100% accurate. But when considering Urban mpg, it’s wise not to fully rely on the number given. That’s because it’s impossible to predict city traffic.

Depending on what’s going on around you, it could take hours to complete a short journey or no time at all, and the Urban Cycle test can’t account for every type of traffic.

What is Extra Urban mpg?

Extra Urban mpg accounts for most roads outside the city; A-roads, B-roads and occasional motorway driving.

It’s usually conducted immediately after the Urban Cycle test so the engine’s warm, which represents most real-life scenarios.

Over the course of 4.3 miles, the vehicle is subjected to steady acceleration and deceleration with speeds of up to 75mph. The average speed for the test is around 39mph, mimicking a casual out-of-city drive.

What is Combined mpg?

So, as you may have guessed, a vehicle’s Combined mpg is the average miles per gallon between its Urban and Extra Urban readings. When considering a vehicle’s mpg, it’s the number most rely on.

If you do a mix of driving; both in and outside built-up areas, it’s the mpg your vehicle should be recording. But if not, there could be a few reasons why.

Why is my mpg lower than expected?

If your miles per gallon recording is lower than the manufacturer’s estimations or what’s stated on our website, it could be for several reasons.

It’s unlikely, but it could be that the numbers you were given are just wrong. It may have been a faulty test or simply human error.

However, more likely is your driving style. As said earlier, it’s impossible to state a guaranteed mpg figure as everyone drives differently.

If you’re throttle-happy or resent cruising in a higher gear, you’re likely to use more fuel – as your engine is working harder.

Likewise, if you’re constantly driving in rush hour, speeding up and slowing down regularly, or idling in long traffic jams, you’re not going to record the most efficient drive. But there are ways to combat a lower than expected mpg.

Ways to improve your vehicle’s mpg

There are actually many ways to improve your vehicle’s mpg. We’ve listed a few below.

Go easy on the accelerator; focus on gradually accelerating rather than flooring it
Use your manufacturer’s recommended fuel type
Don’t carry any extra, unnecessary weight
Avoid drag by taking off roof racks or storage boxes when you’re not using them
Make sure your tyres are inflated to the recommended level and keep on top of them
Fill up in the morning when the fuel is colder to get more for your money
Don’t overuse the air-con
Keep your car serviced for maximum efficiency
Plan your routes before setting off
Try to avoid idling for too long

There are of course more in-depth changes you can make to your vehicle, such as remapping your vehicle’s ECU.

But if you’re looking for quick mpg wins, the 10 above should offer you some improvement.