How to accurately calculate your fuel economy for petrol, diesel, hybrid and electric cars

Tristan Young

21 Sep 2021

Fuel efficiency isn’t the be-all and end-all of motoring, but running costs are a significant factor in car choice. Here’s how to work out accurately how much it costs to power a car.

Trip computers that show information such as average speed, distance travelled and average fuel economy have been present on the majority of cars for several years, but how do you know if they’re accurate?

Ask any experienced road tester any they’ll tell you trip computers are pretty accurate, but not 100% accurate.

If you want to know exactly how much fuel your car is using – and what it’s ultimately costing you per mile – here’s what you need to do.

Petrol & diesel

In order to work out an accurate fuel consumption for a petrol or diesel car – including hybrids, but not plug-in hybrids – you need to start with a full tank of fuel. And we’re talking about properly full. The best way to do this is use the three clicks method. That means filling the car up until the pump clicks off, let the tank settle then do this twice more.

Once you’ve done this, make a note of your car’s mileage.

The point at which you measure the fuel consumption for the first time comes when you next fill up.

Do the same three-clicks brimming of the tank and then make a note of the litres used, the distance between fills (by subtracting the first mileage from the one at the second fill) and the cost of the fuel.

You should end up with something like this:

Litres: 42.30

Miles: 244.3

Cost: £63.83

From these figures you can then work out your fuel consumption. If you want miles per gallon, you’ll need to convert your litres to gallons, so multiply by 0.22.

In this case that gives you 9.306 gallons.

Because you want miles per gallon, you take the miles and divide by the gallons figure.

So, 244.3 / 9.306 = 26.25mpg.

And in case you were wondering, that’s what you get from a 10-year-old Mazda MX-5 2.0 if you drive it quickly.

As for the fuel cost per mile, that’s a similar sum. Cost divided by the number of miles, in this case: £63.83 / 244.3 = 26 pence per mile.

And yes, that’s quite high, but like I said it was a fun few miles and I always use super unleaded in my MX-5.

Electric vehicles

To work out an accurate economy of an electric vehicle is straightforward if you’re always filling up at the same place, for instance, at home.

Again you need to start with full battery and you need to make note of your mileage.

Once the journey is completed, note the mileage again, plug in and charge to full.

The trick here is to have access to your charging information, but most home charge points are connected and you can access the data online or through an app. And in some cases, through the car. To work out the cost per mile, you’ll also need to know your per kWh electricity cost.

You’ll then end up with something like this:

kWh: 46

Miles: 128.8

Cost: 15p/kWh

The sums are then similar to an ICE car. In other words; miles divided by kWh. In this example, that’s 128.8 / 46 = 2.8 miles per kWh, which is what you get from an Audi e-tron 50 on a long motorway cruise.

To get the cost per mile, divide the price per kWh of electricity by the efficiency figure. In this case that’s 15p / 2.8 = 5.4 pence per mile.

Plug-in hybrids

Plug-in hybrids (PHEVs) are the most complicated to work out the true economy for because you’ve got both a petrol (or possibly diesel) engine and an electric motor.

It’s relatively easy to work out an mpg figure using the method described for purely petrol or diesel cars. However, that figure wouldn’t tell the whole story – a bit like the official fuel efficiency figures for PHEVs.

You could easily end up with a figure well above 100mpg if you always plug your car in and only do short journeys.

Calculating a miles per kWh figure would also work in the same way as you would for a full EV. However, because the engine is likely to be providing some of the propulsion, this figure is likely to be high in comparison to a full EV’s figure.

The solution is to work out the cost per mile for the two fuels.

Rather than work this out for a single tank and battery full, it’s easier (and more accurate) to look at the bigger picture of lots of journeys and lots of miles.

You can effectively use the two methods above to provide a cost figure. Total electricity cost plus total fuel cost divided by the total mileage.

As an example, here’s the figures for a Citroen C5 Aircross PHEV.

Over 1,500 miles the car averaged 53mpg. Today’s average petrol price is £1.35 per litre which equates to 11.6p a mile.

Over the same distance the car used £50.63 of electricity so that’s an additional 3.4p a litre, bringing the total cost to 15p a mile.

Armed with an accurate cost per mile allows a level playing-field for comparisons between cars with different powertrains. While motoring isn’t all about cost, it’s a significant factor that can’t be ignored.

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Tristan Young

21 Sep 2021