On fuel cost alone, a detour is worthwhile when the saving on your fill-up exceeds the fuel used to reach the station and rejoin your journey. Calculate the net saving, then decide whether it also justifies your time and other costs.
The formula
Net fuel saving = (fill-up volume × price difference) − (extra distance × consumption / 100 × price of fuel used).
Use litres, kilometres, consumption in L/100 km and the same currency for both prices. Count only kilometres added to your original journey, but include all of them: outbound, return or the route needed to rejoin your journey.
Example: 40 litres and 10 extra kilometres
Assume a 40-litre fill-up, a €0.10/L price difference, 10 extra kilometres in total, consumption of 6 L/100 km and fuel used valued at €1.80/L.
| Step | Calculation | Result |
|---|---|---|
| Gross fill-up saving | 40 × 0.10 | €4.00 |
| Detour fuel used | 10 × 6 / 100 | 0.60 L |
| Cost of that fuel | 0.60 × 1.80 | €1.08 |
| Remaining saving | 4.00 − 1.08 | €2.92 |
That leaves €2.92 before time, tolls and wear. These hypothetical values illustrate the method; they are not current prices.
Which fuel price should you use?
Use a consistent estimate of your vehicle’s fuel cost, such as the price you expect to pay to replace it. This price only values the fuel consumed by the detour. The difference between station prices is a separate input.
If you do not know the actual consumption for this journey, start with an estimate for your vehicle and try a higher value too. The result is sensitive to this assumption, particularly for urban detours.
At what distance does the saving disappear?
When consumption and price are both strictly positive, the maximum extra distance before fuel savings reach zero is: gross saving × 100 / (consumption × price).
For the example above: 4 × 100 / (6 × 1.80), or about 37 km in total. This is a mathematical break-even point, not a recommendation to drive 37 km for fuel. Your time can make the detour unattractive well before that point.
Three checks before leaving
- Actual volume: a small top-up reduces the potential saving.
- The full route: straight-line distance or the outbound leg alone can understate the detour.
- Price freshness: Fillzz refreshes data every 10 minutes, but freshness depends on the information supplied by data providers. A price change can remove the expected saving.
Read about Fillzz data limitations and prepare your fuel stop. In the app, compare stations and check the route before driving.