UK petrol prices: when a cheaper station costs more

By AtheneNet Money Desk

Driving farther for cheaper petrol or diesel only saves money when the pump discount exceeds the fuel cost of the extra journey. Before setting off, compare the exact round-trip distance, your realistic fuel economy, the amount you will buy and current local prices—not just the largest price-board difference.

A useful break-even test is:

Net saving = pump-price saving on the fill − cost of the extra journey

If the result is zero, the trip has only recovered its additional fuel cost. If it is negative, the apparently cheaper station costs more overall, even before allowing for time or vehicle wear.

Calculate the fuel cost of reaching the cheaper station

For a vehicle measured in miles per imperial gallon, calculate the litres consumed on the additional round trip as follows:

Round-trip fuel used in litres = round-trip miles × 4.54609 ÷ mpg

Then calculate the journey cost:

Journey cost = litres used × pump price per litre

Use the additional mileage created by choosing the cheaper station. If it lies directly on a journey you were already making, the relevant distance may be only the detour rather than the full distance from home.

To find the required discount per litre, divide the journey cost by the number of litres you expect to buy:

Break-even discount per litre = journey cost ÷ litres bought

Multiply the answer in pounds by 100 to express it in pence per litre. The calculation estimates fuel expenditure only; it does not prove that making the journey is worthwhile once time and other running costs are considered.

Worked example: a six-mile round trip at 40 mpg

This example is illustrative, not a live UK pump-price report.

The fuel calculation is 6 × 4.54609 ÷ 40, producing approximately 0.68 litres. Multiplying that amount by £1.45 gives a journey cost of approximately £0.99.

Dividing £0.99 by a 40-litre fill gives roughly £0.0247, or 2.5p per litre. The cheaper station therefore needs to undercut the alternative by about 2.5p per litre merely to recover the journey fuel cost.

A 4p-per-litre discount on 40 litres would save £1.60 at the pump. After subtracting the estimated 99p journey cost, the remaining fuel-only saving would be about 61p. A 2p discount would save only 80p at the pump, leaving the driver around 19p worse off on this simplified calculation.

Congestion, fill size and discounts move the break-even point

The same station can be worthwhile on one visit and poor value on another. Factors that change the result include:

• Congestion: slow traffic and idling can reduce real-world fuel economy, increasing the journey cost.
• Cold starts: a short trip from a cold engine may use more fuel than the vehicle’s usual average suggests.
• Vehicle wear: tyres, servicing and depreciation create costs beyond fuel, although the amount varies by vehicle.
• Membership discounts: compare the price you personally qualify for and account for any relevant fee or condition.
• Fill size: buying more litres spreads the journey cost across a larger purchase, lowering the required discount.
• Fuel economy: a less efficient vehicle uses more fuel over the same distance and needs a larger pump saving.

For example, the 99p journey cost spread across 20 litres requires nearly 5p per litre to break even. Spread across 50 litres, it requires about 2p per litre. Use the litres actually purchased, not the tank’s stated capacity.

Check current local prices before making the trip

Pump prices vary by location, fuel grade, station and time, so the £1.45 figure above should not be treated as a current national average. Check the prices displayed by nearby forecourts or a current local comparison service immediately before travelling, and confirm that both quotes cover the same fuel grade and discount conditions.

RAC Fuel Watch provides current UK fuel-price monitoring and consumer context. The Competition and Markets Authority’s road-fuel market study provides wider background on competition and pricing in the UK road-fuel market.

For a quick decision, note four figures: the additional round-trip miles, realistic mpg for that journey, litres you will buy and the live price difference. Recalculate if traffic, route or purchase size changes.

Source: RAC