How do I work out what my car’s real mpg is?

real mpg

Every car with petrol, diesel or hybrid power covers a certain number of miles per gallon (mpg). When you’re looking at new cars, the manufacturers give an mpg – the number of miles a car covers on a single gallon (4.55 litres). The higher the mpg number the less fuel you’ll use.

However, the way car makers calculate their mpg doesn’t really reflect everyday driving. But you can still calculate mpg accurately yourself.

What do you need to calculate your mpg?

If you’re old skool, get a note pad and pen. These will live in your car while you’re calculating your mpg. Split the page up into four columns: litres, cost, mileage and trip.

Alternatively, you can keep a note of those four things somewhere on your mobile phone or in an app such as Gas Manager. The most important factor is that it’s easy and you don’t forget to do it or you’ll throw all your careful calculations out.

How to work out your mpg

First fill your car to the brim. Then before you drive off, record your car’s overall mileage and zero the trip. You don’t need to put in the amount of fuel it’s got this first time because theoretically it will be full. The next time you fill up, write in exactly how much you’ve put in, the cost and mileages. Do this every time you stop for fuel.

Depending on the miles you cover, you may want to calculate your car’s mpg after weeks or months. In many ways, the longer you do this for, the more accurate a reflection of your motoring and therefore mpg you’ll get.

The calculation

1: Add up the number of litres of fuel you’ve bought

2: Multiply the total litres by 0.219 to convert it to gallons

3: Subtract the starting mileage from the final mileage (this should equal the trip read out)

4: Divide the number of miles by the number of gallons you’ve put in

5: Divide the total cost by the number of miles for your fuel cost per mile

real mpg
A gratuitous picture of one of the most beautiful cars ever built: Ferrari 246GT. Should you be lucky enough to own one, you could use the above calculation to work out its mpg

If you drive a plug-in hybrid…

The above calculation will give you an accurate figure for the amount of fuel you’ve put into the car and its cost. But to get the overall cost for a plug-in hybrid, you need to account for how much electricity you’ve used too. To do this you must calculate the increase to your electricity bill every month from the norm and add that in. Unfortunately, it’s harder to be any more accurate.

The official calculation

We’ve already covered what the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) used by car makers is all about. It’s a better solution than the New European Driving Cycle (NEDC) that was used from the 1980s until 2017. But it’s still not perfect.

WLTP is lab-based like NEDC. But it’s very difficult to replicate real world conditions such as wind direction and gradient so that every car is tested in identical conditions.

The easy way to check your car’s mpg

The majority of modern cars have trip computers. These display miles per gallon and there’s often the ability to show it long-term or just for your most recent journeys. However, these usually aren’t 100% accurate and have a small tolerance (around 3%). And while this indicates your mpg, it won’t help you to calculate the cost of fuel which doing it yourself does.