Will a bank-holiday fuel detour really save you money?

A cheaper pump is worth the bank-holiday detour only if the saving on the fuel you buy exceeds the cost of reaching it. Compare the extra round-trip fuel cost with the price difference across your planned fill—not merely the two displayed prices.

By AtheneNet Money Desk

National averages show the trend, not your cheapest forecourt

The Department for Energy Security and Net Zero publishes weekly UK average pump prices for unleaded petrol and diesel. This series can show whether prices are generally rising or falling, but it cannot identify the cheapest station near a particular driver.

RAC Fuel Watch also tracks petrol and diesel price movements and provides consumer context. Both sources are useful benchmarks, but a weekly national figure should not be treated as a live local quote.

For an actual journey decision, open a current postcode-based fuel-price service and compare nearby forecourts. Check when each quote was updated, whether it covers petrol or diesel, and whether the price requires membership, an app or another condition. Pump prices can change before a database catches up, so the forecourt’s displayed price remains decisive.

In practical terms:

• Use the government series to understand the national weekly direction.
• Use RAC Fuel Watch for wider pump-price context.
• Use time-stamped local quotes to compare reachable forecourts.
• Recalculate if the route, purchase volume or displayed price changes.

Calculate the extra cost of reaching the cheaper pump

First, measure only the additional distance. If the filling station is already on your route, the extra mileage may be negligible. If visiting it creates a separate journey, count the full additional round trip.

Estimate the fuel consumed using your car’s realistic fuel economy rather than its best laboratory figure. For miles per gallon, the calculation is:

Additional litres used = extra miles ÷ mpg × 4.546

Then calculate the journey cost:

Detour cost = additional litres used × expected fuel price per litre

Finally, calculate the gross pump saving:

Pump saving = litres purchased × price difference per litre

The detour saves money only when the pump saving is greater than the detour cost. To find the minimum price gap needed, divide the detour cost in pence by the number of litres you expect to buy.

Break-even gap in pence per litre = detour cost in pence ÷ litres purchased

Illustrative example with changeable assumptions

This example is not a universal result. Replace every assumption with the distance, fuel economy, purchase volume and local quotes that apply to your journey.

Here, an eight-mile detour uses approximately 0.81 litres: 8 ÷ 45 × 4.546. At £1.45 per litre, that journey costs about £1.17. Buying 40 litres at 4p per litre less saves £1.60 before travel, leaving roughly 43p.

The break-even price difference is about 2.9p per litre, calculated as 117p divided by 40 litres. A smaller gap would not cover the estimated detour cost. Buying fewer litres raises the required gap; combining the fill with an existing journey can lower it.

Small real-world costs can erase the saving

The calculation is an estimate because fuel consumption changes with conditions. Bank-holiday congestion, queues at the forecourt, repeated acceleration, roof boxes, heavy loads and air-conditioning can all increase consumption.

Underinflated tyres or poor tyre condition may also reduce efficiency and introduce a safety concern that matters more than a marginal saving. Use the manufacturer’s recommended tyre pressures and never extend a trip solely to chase a tiny price difference when the vehicle needs attention.

Driving style matters too. Smooth acceleration and anticipating traffic generally produce a more realistic result than relying on an optimistic dashboard average. If the cheaper station requires an unnecessary journey, include the entire trip rather than mentally treating those miles as free.

Before leaving, run the calculation with the latest local quote and add a modest buffer for traffic or stale price data. If the estimated net saving is only a few pence, waiting until the cheaper forecourt is naturally on your route may be the better household-budget choice.

Source: Department for Energy Security and Net Zero