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Why mileage matters when buying a car

Running costs are essential to consider when buying a car, and fuel economy is big part of running costs. After all, petrol and diesel aren’t getting any cheaper, so the fewer trips to a filling station, the healthier your wallets are going to be.

We explain why understanding that a car’s mileage, measured in ‘miles-per-gallon’ or ‘mpg’ figure can help you understand the implications for your fuel bills and make the right choice of car.

What is mpg and what does it mean?

The mpg figure simply refers to the number of miles you can drive using one gallon of fuel.

As a rule of thumb, an efficient car will do more than 60 miles per gallon. Anything over 50 miles per gallon can be regarded as decent fuel consumption. If you’re getting below 25 miles per gallon, efficiency probably isn’t your priority.

Assessing a car’s mpg – where do the numbers come from?

Cars actually use different amounts of fuel depending on the type of roads they drive on. Stop-start urban driving uses up more fuel than going at a steady speed on a motorway, for example.

All cars are tested before going on sale to measure their fuel economy. There are different figures for urban driving and ‘extra-urban’ (motorway-style) driving, with a combined figure giving an idea of what the average miles per gallon for a vehicle should be. 

These official figures have been criticised for many years for being unrealistic, because they were calculated using laboratory tests. However, since September 2018 there’s a new test based on real-world testing, which, is proving to be more accurate. It’s called the WLTP which stands for the snappily titled Worldwide Harmonised Light Vehicle Test Procedure.

A number of motoring websites have also been conducting their own tests for a while, revealing a car’s ‘real mpg’ or ‘true mpg’. Take a
look at these, as they are a good indicator.

Does mpg change as the car gets older?

Some people believe that a car’s fuel economy deteriorates as it gets older, but this is a myth. A used car will still return new-car mpg figures as long as it has been well maintained. If a car has a full service history with lots of entries, it should deliver the same mpg as when it was new.

How can I have an effect on my car’s mpg?

Of course, a car’s mpg also depends on the driver. If you drive a car economically, you should be able to attain the true or real mpg figures – and even the new official WLTP ones.

There are lots of ways to be economical when driving:

  • Don’t rev the engine too much. Stick to around 2,000rpm on the gauge: if you reach it, change up a gear. 
  • Minimise your braking. Anticipate when you might need to slow down and lift your foot off the accelerator in plenty of time, just using the brake if you need to slow down quicker, or come to a stop. 
  • Drive smoothly, avoiding harsh braking and accelerating. 
  • Reduce the weight of your car. Don’t fill your boot with stuff you don’t need – and don’t fit a roof rack or roof box when it isn’t needed. 
  • Check regularly that your tyres are correctly inflated (as per the car’s manual). 
  • Don’t use the car’s air conditioning if you don’t need to.
Calculating your mpg

Most cars today have a trip computer that will tell you what your ongoing mpg figure is, as well as what it is for your trip and the overall figure since you last reset it. This is a good guide to what you’re managing to achieve – and easier than working it out yourself.

Using mpg and litres

We measure a car’s fuel economy using mpg: that is, how many miles can we drive using one gallon of petrol or diesel. 

But just to complicate things, fuel is sold in litres, not gallons. One gallon is 4.5 litres, so the official mpg figure is also miles per 4.5 litres.

If you divide the car’s mpg figure by 4.5, you can see how many miles your car will drive on every litre of fuel. That means if the car is supposed to do 45mpg, it will do 10 miles per litre (45 divided by 4.5). And if fuel is, say 125p per litre, then you know that it will cost you £1.25 for your car to go 10 miles.

Then look up the capacity of your car’s fuel tank – in the car’s manual, or a quick online search – in litres. Multiply that by your figure for miles per litre and you’ll know how far your car will drive on a full tank (its range). If you have a 40-litre tank, and you can go 10 miles per litre, you should be able to drive 400 miles on a full tank.

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