What Does the Cheaper Fuel Station Detour Calculator Do?
A cheaper pump price does not automatically mean a cheaper fill. Reaching that station can consume part of the saving. This calculator compares the price reduction on the litres you buy with the extra fuel used by the detour, giving a net fuel saving and the price difference needed to break even. It is useful for deciding whether a small advertised discount is meaningful for the fill you actually intend to make.
How to Use This Calculator
- Enter the usual station's price and the alternative station's price in pence per litre, using the same fuel grade. A comparison between different grades answers a different question.
- Enter the litres you genuinely expect to buy. Use the intended purchase, not the tank's maximum capacity when it still contains fuel. The discount applies only to litres bought on this visit.
- Enter the additional round-trip miles caused by choosing the alternative. If it lies on a journey you are already making, include only the extra distance beyond that existing route.
- Enter realistic UK MPG for that extra driving and calculate. Read gross saving and extra fuel cost before judging the net figure. Consider time and any separate charges alongside the result.
The Maths & Assumptions Behind the Tool
Gross saving = litres bought × (usual pence − alternative pence) ÷ 100. The model prices detour fuel using the usual pump price: extra miles ÷ MPG × 4.54609 × usual pence ÷ 100. Net saving is gross saving minus that cost. Dividing extra fuel cost by the litres bought and converting to pence gives the pump-price difference needed to cover the detour. The result values fuel only, not your time.
Worked example
A 45-litre fill at 140p instead of 145p saves £2.25 at the pump. An extra 8 miles at 40 UK MPG uses 0.9092 litres. Valuing that fuel at the usual 145p costs £1.3184. Net saving is therefore £0.93 before valuing your time or wear.
The price gap must exceed 2.93p/litre to cover this detour’s fuel. Enter the total extra mileage of the detour, including the return to your route, rather than just the distance to the cheaper station.
Why This Matters for UK Drivers
The key distance is extra mileage compared with the route you would otherwise take. If a station is already on the way, the additional distance can be close to zero. If reaching it and returning adds eight miles, enter eight rather than the four miles from your original route to the forecourt. The litres input is the amount you expect to purchase, not necessarily the tank's advertised capacity.
The arithmetic values detour fuel at the usual station's price and excludes time, tyre wear, parking and other costs. A positive result of a few pence can therefore be too small to justify a separate journey. If you are filling only a small amount, a modest discount has little opportunity to cover the detour. Combining a fill with an existing shopping journey can change the answer because the relevant additional distance becomes smaller.
Interpreting the result in practice
A fuel-saving detour can become marginal quickly for a small top-up. Buying twice as many litres doubles the gross price saving, while the extra journey uses roughly the same modelled fuel. This is why the same station can be worthwhile for one fill and unattractive for another. A queue, idling or an extra stop may also use more fuel than a smooth-distance estimate captures.
For a sensible decision, use a price you can reasonably expect to find on arrival and a safe planned route. A displayed saving of a few pence is easily erased by a small distance or economy error. If you would visit the area anyway, allocate only the additional fuel-station diversion; charging the entire unrelated journey against the cheaper fill would answer the wrong question.
Frequently asked questions
What should I enter for extra distance?
Only the miles you would not otherwise drive. If the cheaper station is directly on your normal route, enter zero.
Does this include the value of my time?
No. If the detour takes significant extra time, the true benefit may be even smaller.
Which fuel price is used for the detour?
The calculator uses your usual local price as a conservative estimate of the fuel burned getting there and back.
Looking past the pump-price headline
What the number tells you
The cheapest fuel sign is not automatically the cheapest fill. Driving extra miles consumes fuel before you reach the cheaper station and again on the way back to your route. This calculator values that detour so you can see the net saving rather than only the pence-per-litre difference.
Use realistic inputs
Be careful with the detour distance: enter the extra mileage caused by the cheaper station, not the full distance from home. If the station is already on a journey you are making, the extra distance may be close to zero. If it requires a dedicated round trip, include the whole additional distance.
What is outside the calculation
Time and wear are not given a cash value here. A small positive fuel saving may still be unattractive if the detour adds congestion, parking hassle or meaningful travel time. The tool is best used to identify whether the fuel-price difference is large enough to matter at all.