UK petrol costs: work out your journey price before choosing a route

A journey’s petrol cost depends on three things: the distance you drive, the fuel economy your car achieves on that trip and the price per litre where you can actually fill up. Calculate each route separately before choosing one. A shorter route may use less fuel, while a longer route could still be cheaper if your car runs more efficiently and you can buy petrol at a lower price.

Calculate petrol cost from distance, fuel economy and pump price

Start with the route distance in miles. Use the total distance you expect to drive, including a return trip if you are budgeting for one. Then find a realistic fuel economy figure in miles per gallon (mpg). Your car’s trip computer or past fill-ups may give you a better estimate for your driving than its advertised figure.

UK mpg uses imperial gallons. One imperial gallon is about 4.546 litres, so the calculation is:

Distance in miles ÷ mpg × 4.546 × pump price in pounds per litre = estimated petrol cost.

If the pump displays pence per litre, divide that number by 100 first. For example, a hypothetical 145p per litre becomes £1.45 per litre. Keep the same units throughout: mixing pence with pounds, or imperial gallons with US gallons, will give the wrong result.

Some vehicles show consumption in litres per 100 kilometres (L/100 km). For those, use: distance in kilometres ÷ 100 × L/100 km × price per litre. You can also use this version if your route planner gives kilometres and your car displays L/100 km. Avoid switching between the two methods halfway through.

Worked example: a hypothetical 120-mile trip

Suppose a driver is comparing a 120-mile route, expects to average 45 mpg and can buy petrol for a hypothetical £1.45 per litre. These figures illustrate the method; £1.45 is not a quoted current UK price.

• Fuel used: 120 ÷ 45 × 4.546 = about 12.12 litres.
• Petrol cost: 12.12 × £1.45 = about £17.58.

That is an estimate of the petrol the journey will consume. It is not necessarily the amount the driver will pay at the pump that day. Someone who starts with a full tank may make no purchase during the trip, but the journey still uses fuel that will eventually need replacing.

For a return journey over the same route, doubling £17.58 gives a starting estimate of £35.16. Recalculate if the return route, traffic, fuel economy or available pump price is likely to differ. A small margin in the budget can help when conditions are uncertain.

Compare routes using the prices you can actually reach

A national average is useful context, but it is not the price a particular driver is guaranteed to pay. Before leaving, check a current price at a station on or near each route. RAC Fuel Watch provides UK fuel-price monitoring and context; for a route decision, also check the price shown by the station where you intend to fill up. Prices can change, so confirm the displayed pump price before buying.

Run the calculation for each route with its own distance, expected mpg and available price. For comparison, imagine a second, hypothetical route of 130 miles. If the car averages 40 mpg on that route and petrol available to the driver costs £1.55 per litre, the estimate is 130 ÷ 40 × 4.546 × £1.55, or about £22.90. Against the first route’s £17.58, that is about £5.32 more in petrol. Both pump prices are invented examples, and the comparison changes if either route’s real conditions differ.

Motorway services and local forecourts may display different prices. Their locations, operating costs and competitive conditions can differ, and a lower advertised local price is only helpful if reaching it makes sense for the journey. Include any detour in the distance calculation. If you already have enough petrol to reach a cheaper station safely, you can compare the cost of filling there; do not assume that station will be accessible from every route or open when you arrive.

Allow for driving conditions before deciding

Fuel economy is an estimate, not a fixed property of a route. Sustained high speeds, stop-start traffic, hills, heavy loads and poor weather can change how many litres you use. A direct route through congestion may return worse mpg than a slightly longer route with steady traffic. If you have trip-computer records for similar journeys, use them to test whether the mpg figure in your calculation is realistic.

When the predicted difference between routes is small, try a second calculation with less favourable mpg. For instance, if a delay or a faster road reduces your expected economy, substitute that lower mpg while keeping the route distance and relevant pump price the same. This gives you a useful range rather than a false sense of precision.

The petrol calculation also leaves out parking, tolls and the value of your time. Add those separately if they matter to your choice. Before setting off, check the route distance, a plausible mpg figure and the pump price at a station you can use. Those three inputs will give you a more useful journey estimate than an older national average.

Source: RAC