UK bank holiday fuel maths: 5p gap saves £2.50

A petrol station charging 5p less per litre cuts the cost of a 50-litre fill by £2.50. A 10p gap saves £5. Those figures can help bank holiday drivers compare pumps, but the cheaper displayed price only pays off if any extra journey costs less than the saving.

By AtheneNet Money Desk

Turn the price gap into a real fill-up saving

The calculation needs only two numbers: the difference between the pump prices and the number of litres purchased.

Saving = price gap in pounds × litres purchased

For example, a 5p difference is £0.05. Multiply £0.05 by 50 litres and the saving is £2.50:

£0.05 × 50 = £2.50

The same method works for any purchase. If one station is 7p cheaper and the driver buys 35 litres, the saving is £0.07 × 35, or £2.45.

This comparison does not identify the cheapest station or show what petrol and diesel cost today. It simply converts a visible pump-price difference into a cash amount.

Check whether the cheaper station is worth the detour

A lower price can be cancelled out if reaching it requires extra fuel. Use the additional round-trip distance—not the full distance already planned—and compare its fuel cost with the fill-up saving.

For a car measured in UK miles per gallon:

1. Divide the extra round-trip miles by the car’s realistic mpg.
2. Multiply the result by 4.546 to convert UK gallons to litres.
3. Multiply the litres consumed by the fuel price in pounds.
4. Subtract that detour cost from the pump-price saving.

An illustrative 10-mile detour in a car achieving 40mpg uses about 1.14 litres. At an assumed £1.50 per litre, that fuel costs roughly £1.70. A £2.50 fill-up saving would therefore fall to about 80p net.

The example price is an assumption, not a current UK average. Drivers should replace it with the price displayed at the pump and use fuel economy that reflects their own car, load and likely traffic.

A shorter calculation is possible when the car shows a reliable miles-per-litre figure:

Detour cost = extra round-trip miles ÷ miles per litre × pump price

Time, parking, congestion and vehicle wear are separate costs. A detour that produces a few pence of net fuel saving may still be poor value during a busy bank holiday journey.

National averages provide context, not a local quote

The UK Government publishes weekly road-fuel price statistics covering petrol and diesel. These series are UK national averages, so they should not be treated as the price available at a particular forecourt.

The publication date and latest numerical averages were not included in the source evidence supplied for this calculation, so no live national figure is quoted here. Readers can check the government’s weekly oil and petroleum statistics page for the latest dated release.

RAC Fuel Watch separately provides consumer-facing information and commentary on UK petrol and diesel prices. Its figures and observations can help explain the national picture, but they do not override the amount displayed at the chosen pump.

The pump display decides the actual saving

Motorway service stations, local forecourts and membership schemes can charge different prices. Some offers may also depend on a card, app or eligibility condition.

Before changing route, compare the same fuel grade at both locations and confirm that any advertised price is available to you. Multiply the genuine per-litre gap by the litres you expect to buy, deduct the extra round-trip fuel cost, and use the remaining amount—not the headline price difference—to make the decision.

Source: UK Government