Fuel efficiency calculations

Some maths to try and understand the comparative fuel efficiency of vehicles using petrol, diesel and electricity as their fuel. For example, is a 3 mile per kWh electric car more or less fuel efficient than a 80 mile per gallon diesel car? (Spoiler: more)

First we need to understand the energy content of the fuels in question. According to the UK Government there is 9.6kWh of energy in a litre of petrol and 10.9kWh in a litre of diesel. We can then work out the miles per kWh by dividing the miles per gallon figure by 43.5 for petrol cars, and by 49.5 for diesel cars.

So a 50 mpg diesel car is doing roughly 1 mile per kWh. And a 22 mpg petrol car is achieving 0.5 miles per kWh. Almost all electric cars can manage at least 2.5 miles per kWh, so a diesel car would have to do 123.75 mpg to be using its fuel as efficiently as that, and for a petrol car it would be 108.75 mpg. Electric motorbikes can usually achieve twice the fuel efficiency of an electric car, so those figures would be double.

Why is this? Optimistically only about 35% of the actual energy in diesel and petrol is converted into motion, the rest is mainly wasted as heat from combustion of the fuel. Meanwhile electric motors generally turn over 90% of their fuel into motion, with the batteries wasting energy as heat at the extremes of their performance envelope.

Raw fuel efficiency is one aspect, but fuel cost is equally important, i.e. £ per mile. Right now in our part of the UK the cost of petrol is about £1.20 per litre, and diesel about £1.24 per litre. Electricity can cost up to £0.30 per kWh from a commercial fast charger, or as little as £0.045 per kWh on an EV-friendly domestic energy tariff.

To work out the £ per mile we simply divide £5.45 by the fuel efficiency of a petrol car, or divide £5.63 by the fuel efficiency of a diesel car. So our 50 mpg diesel car costs £0.113 per mile, and the 22 mpg petrol car costs £0.248. The electric car calculation is the same, divide the cost of a kWh, say £0.30, by the fuel efficiency, say 2.5 miles per kWh, and in this case get £0.12 per mile.

Purely on fuel costs, a 50 mpg car filled with £1.24 per litre diesel is cheaper to run than a 2.5 mile per kWh electric car charged with £0.30 electricity.

However, charge the electric car overnight at home at £0.045 per kWh and the £ per mile drops to 1.8 pence per mile. Almost a tenth of the £ per mile of the diesel car. And the electric motorbike mentioned above is going to easily cost less than a penny per mile to run.

Finally, if the electricity you charge your car or motorbike with has come from the solar PV panels on your rooftop, then the pure fuel cost is zero...