Fuel Cost vs CO2 Comparator

This comparator puts four fuels side by side — petrol, diesel, LPG, and electric — on the two metrics that matter for running costs and policy: cost per mile and CO2e per mile. You enter each vehicle’s efficiency, the relevant prices, and a distance, and it ranks them with a UK fuel-duty and VAT breakdown.

How it works

For liquid fuels, miles per gallon is converted to miles per litre, then cost and CO2 follow from price and emission factor per litre. The EV works from miles per kWh and the grid carbon intensity:

mpl          = mpg / 4.546                 (UK gallon = 4.546 L)
cost_per_mile = price_per_litre / mpl
co2_per_mile  = factor_per_litre / mpl
EV: cost_per_mile = price_per_kWh / miles_per_kWh
    co2_per_mile  = grid_factor / miles_per_kWh

DEFRA tailpipe factors are about 2.31 kg CO2 per litre of petrol, 2.51 for diesel, and 1.51 for LPG. The tax line splits the pump price into fuel duty plus 20 percent VAT charged on top of the duty-inclusive price.

Understanding the UK pump price breakdown

When you pay at the pump in the UK, a substantial portion goes to the government rather than the fuel itself. Fuel duty is a fixed-rate levy charged per litre before VAT is applied. VAT is then charged at 20% on the whole price including the duty, which means you pay tax on the tax. For petrol and diesel, the tax component of the pump price has historically represented over half the total cost at typical pre-tax retail prices.

For LPG, a lower duty rate applies because it is used partly as a transport fuel and partly for heating. Electricity for road charging carries no fuel duty but does attract VAT at 5% for domestic charging and 20% at public rapid chargers — a notable disparity that affects the real-world cost comparison between home and public charging.

Illustrative comparison example

Consider these typical inputs for a UK driver covering 12,000 miles per year:

These are illustrative figures — enter your own vehicle’s actual efficiency and your local prices for an accurate comparison.

Over 12,000 miles, the difference between the petrol and EV rows in this example represents roughly £1,400 per year in fuel savings. Whether that offsets the purchase price difference depends on the vehicles chosen and how long they are kept.

The EV emissions question

An EV emits no tailpipe CO2, but it is not zero-carbon in operation — the carbon is upstream at the power station. UK grid carbon intensity has fallen substantially as renewables have grown, and on a grid-average basis an EV typically produces significantly less lifecycle CO2 than an equivalent petrol car. However, the comparison changes if you charge mostly from a coal-heavy grid or if you compare only against a highly efficient diesel. Set the grid intensity slider to your best estimate of your real supply — a renewable tariff, for example, justifies a much lower figure than the national average.