UK Fuel Prices: Will a Bank-Holiday Detour Save Money?

A filling station charging 4p less per litre saves £1.60 on a 40-litre purchase. However, if reaching it requires an eight-mile extra round trip, the fuel used could consume about £1.32 of that saving under the illustrative assumptions below. The apparent bargain would then be worth only 28p before considering time or vehicle wear.

A lower pump price does not guarantee a cheaper journey

The relevant comparison is not simply one pump price against another. Drivers need to compare the saving on the planned purchase with the cost of travelling any additional distance.

Only count miles added to the journey. If the cheaper forecourt is already on the bank-holiday route, the extra distance could be negligible. A dedicated trip from home and back creates a larger hurdle, especially when buying only a small amount of fuel.

Keep weekly national averages separate from local quotes

The Department for Energy Security and Net Zero publishes a weekly UK series covering average unleaded petrol and diesel pump prices. It is useful for seeing the national trend and comparing broad movements between the two fuels.

Those figures are weekly national averages, not prices available at every forecourt. They cannot establish which nearby station is cheapest or guarantee what a driver will pay on a particular day.

RAC Fuel Watch also tracks petrol and diesel price movements and provides consumer context. For an actual journey decision, check a current local-price service and note when each quote was recorded. Compare the same fuel grade and identify any membership, app or payment conditions.

A live local listing is still a quote rather than a guarantee. The displayed price may have changed, so a roadside price board or direct forecourt confirmation is the final check.

Calculate the detour break-even point

Use these four steps:

1. Find the additional round-trip distance caused by choosing the cheaper station.
2. Estimate how much fuel the car will consume over that distance.
3. Multiply the extra fuel by the expected local price per litre.
4. Compare that detour cost with the saving across the litres being purchased.

For a car measured in UK miles per gallon:

Extra fuel in litres = extra miles ÷ mpg × 4.546

For a car measured in litres per 100 kilometres:

Extra fuel in litres = extra kilometres × L/100km ÷ 100

Then calculate:

Detour cost = extra fuel × expected price per litre

Pump saving = litres purchased × price difference per litre

Net saving = pump saving − detour cost

The minimum price gap needed to break even is:

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

Illustrative example with changeable assumptions

This is a worked illustration, not a current price quote or a universal result.

The extra journey would use approximately 8 ÷ 40 × 4.546 = 0.91 litres. At £1.45 per litre, that fuel costs about £1.32.

The cheaper pump saves 40 × £0.04 = £1.60. Subtracting the detour cost leaves an estimated net fuel saving of 28p.

In this example, the break-even gap is about £1.32 ÷ 40 = 3.3p per litre. A gap smaller than that would not cover the additional fuel. Change every assumption to match the actual route, vehicle and intended purchase.

Congestion and vehicle condition can erase a small saving

Published fuel economy is only a starting estimate. Bank-holiday queues, idling, repeated acceleration and a cold engine can increase consumption. Driving speed and harsh acceleration also affect the result.

Tyres that are underinflated or in poor condition can reduce efficiency and should be checked against the vehicle manufacturer’s guidance. Do not make a journey solely to test fuel economy or chase a marginal price difference.

The calculation should also reflect:

• The amount that will actually fit in the tank
• Petrol or diesel grade differences
• Member-only or payment-specific discounts
• Diversions, roadworks and likely congestion
• Whether the forecourt is already on a necessary journey
• Time and vehicle wear, which the fuel-only result excludes

Before changing route, refresh the local quotes, check their timestamps and recalculate using the extra round-trip distance rather than the full journey. A saving of only a few pence can disappear with one queue or an unnecessary trip.

Source: Department for Energy Security and Net Zero