Every debate about electric cars starts with the purchase price and stops there. But once a car is on your driveway, the number you actually feel is cost per mile — and that's where petrol and electric diverge sharply. Working it out takes two inputs per car: how far it travels on a unit of energy, and what that unit costs.
The universal formula
Cost per mile is energy price divided by efficiency. For petrol that's the price per gallon (or litre) divided by miles per gallon. For an EV it's the price per kilowatt-hour divided by miles per kWh. Same shape, different units — which is exactly why comparing them by instinct fails.
- Petrol: £6.00 per gallon ÷ 40 mpg = 15p per mile.
- EV: 28p per kWh ÷ 3.5 miles per kWh = 8p per mile.
- Charging on a cheap overnight tariff at 8p/kWh drops the EV to about 2.3p per mile.
Efficiency is the lever, not just price
Doubling your mpg halves your fuel cost just as surely as halving the fuel price. Gentle driving, correct tyre pressure and less idle time all move the efficiency term — the one you actually control.
Where the EV advantage comes from
Electric motors convert most of their energy into motion, while petrol engines lose a large share to heat. Combine that efficiency with electricity often being cheaper per unit of usable energy — especially on a time-of-use tariff — and the per-mile cost typically lands well below petrol. Public rapid charging narrows the gap; home charging widens it.
Fuel Cost Calculator
Plug in your own fuel price and mpg to get your real cost per mile and per journey.
Don't forget the break-even distance
A lower cost per mile only pays back a higher purchase price over distance. Divide the price difference between two cars by the per-mile saving to find how many miles you must drive before the cheaper-to-run car comes out ahead. High-mileage drivers reach that point quickly; occasional drivers may never get there.
Compare like for like
Use the same journey and the same driving style for both cars when you compare. A motorway figure for one and a city figure for the other will mislead you every time.
Cost per mile turns a heated opinion into arithmetic. Measure the efficiency, know your energy price, and the right choice for your mileage usually becomes obvious.