Running costs are essential to consider when buying a car, and fuel economy is a big part of those costs. After all, petrol and diesel aren’t getting any cheaper - the fewer trips to a filling station, the healthier your wallets will be.
Here's why it’s important to understand a car’s mileage (measured in ‘miles-per-gallon’ or ‘mpg’) to get to grips with your potential fuel bill, and make the right choice for your next car.
Buying a used car – does mileage matter?
The other way you’ll usually be thinking about mileage when buying a car is if you’re buying a used model. You’ll want to know how many miles that car has covered in its lifetime, as this will likely affect the cost of maintenance.
Mileage is relevant to the car’s age – older cars are likely to have higher mileage, whereas newer models might not have racked up so many.
When it comes to the maintenance of your used car, there are parts that will likely need replacing depending on how high the mileage is. This is a guideline, so it’s a good idea to make sure your car has been thoroughly checked before you purchase.
You might find that some car parts will need replacing after covering a fair few miles:
Tyres often need replacing every 20,000 miles.
Brake pads might need to be changed every 20,000 miles, with 40,000 for the brake discs.
The clutch in your car might need replacing after 100,000 miles.
Choosing a used car with high mileage will often be cheaper upfront, but might cost more in terms of maintenance. Lower-mileage cars are usually newer and more expensive to buy, but will likely be cheaper to upkeep.
What is a good mileage for a used car?
On average, most cars will cover around 10,000 miles per year, but this will obviously change depending on the driver. Using this guideline, you can work out if your car is hitting the average amount of miles for its age or whether it’s covered more.
If you’re looking to buy a used car that’s five years old, you will expect that car to have done around 50,000 miles. Any less than that is a bonus, while any more than that and you might consider the potential wear and tear.
The used car model that you choose will also make a difference, as some manufacturers are considered to be more reliable and have a longer shelf life. It’s a good idea to do your research before you purchase your 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 60mpg. Anything more than 50mpg can be regarded as decent fuel consumption, but if you’re getting less than 25mpg, efficiency probably isn’t your priority.
If you want to skip out on miles per gallon completely and focus on another way of powering your car, an electric car might be a better choice.
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 more fuel than going at a steady speed on a motorway, for example.
All cars are tested to measure their fuel economy before going on sale. There are different figures for urban driving and ‘extra-urban’ (motorway-style) driving, with a combined figure giving an idea of what the average mpg 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, a new test was introduced in September 2018 based on real-world testing, which is proving to be more accurate – the Worldwide Harmonised Light Vehicle Test Procedure (WLTP).
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 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.
This is great news, as used cars are usually cheaper the older they get. This way, you can get an efficient car at a cost-effective price.
What affects my car’s mpg?
A car’s mpg also depends on the driver, of course. 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, only 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.
How to calculate 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.
However, if you own an older car and wish to work out the mpg, it can be done by converting the number of litres into gallons once you've fuelled up.
Multiply the number of litres you filled your car up by 0.219 to get gallons
Subtract the starting mileage with the final number
Divide the number of miles you covered by the gallons of fuel you put in