The simple model
A practical saving estimate starts with the difference between the expensive option and the cheaper option, then subtracts the cost of reaching the cheaper option.
This is the core of Route & Fuel: a fuel price is only useful when it is turned into a route decision. A station that is 4p per litre cheaper can be a great choice for a large fill on the way home, but a poor choice for a small top-up across town.
The formula
Gross saving in pounds = price difference in pence per litre × litres bought ÷ 100.
Detour cost = extra miles ÷ your MPG × 4.546 × fuel price per litre.
Saving after extra driving = gross saving − detour cost.
The 4.546 figure converts UK gallons to litres. MPG is still the way many UK drivers think about real-world consumption, so the conversion keeps the calculation understandable while still pricing the extra fuel in litres.
Example
Say your usual station is 150.9p per litre and a nearby alternative is 146.9p. You plan to buy 50 litres, so the price difference is 4p per litre.
The gross saving is 4p × 50 litres = 200p, or £2.00. If the cheaper station adds 2.5 miles and your car is doing 38mpg on that route, the extra fuel is roughly 0.30 litres. At 146.9p per litre, the detour fuel costs about 44p. The saving after extra driving is therefore about £1.56.
That result may be worthwhile if the stop is convenient. If the same detour is slow, stressful or only saves 30p after fuel, most drivers would reasonably ignore it.
Why your vehicle matters
Two drivers can see different real savings for the same station. A vehicle with different MPG, fuel type or tank size can change whether the stop is worthwhile.
Efficient-driving guidance from the Vehicle Certification Agency and Energy Saving Trust is relevant here because the detour is not just a distance. Harsh acceleration, braking, excessive speed and under-inflated tyres can all worsen real fuel consumption. The same station can therefore be a better deal on a smooth route than on a stop-start route.
What numbers should you use?
- Litres: use the expected fill amount, not always a full tank. A small top-up rarely justifies a long detour.
- MPG: use a realistic recent MPG from your car, not only the best number you have ever seen.
- Distance: count the round-trip extra distance from your actual route, not the straight-line distance to the station.
- Fuel price: use the current station price and check when the price last changed before buying.
- Time: decide your own threshold. Fuel maths can show the cash result, but it cannot decide what ten extra minutes is worth to you.
A quick decision rule
If the gross saving is less than £1, the cheaper station usually needs to be very close or already on your route. If the gross saving is £3-£5, a short detour may be worth checking. If the gap is larger, the route still matters, but the saving has more room to survive the extra fuel cost.
That is why Route & Fuel treats the route as part of the price. The lowest station price is the start of the calculation, not the answer.
Why Route & Fuel is different
Instead of leaving drivers to do this mentally, Route & Fuel is designed to put price, route, detour and MPG details together in the app.
Fuel prices are only the starting point. Your vehicle and journey decide whether the cheaper stop is useful. Route & Fuel combines both so you can avoid a low station price that does not actually save money.