Fuel detour: the simple formula to know if it pays

Decision question

Does the saving at the pump beat the time and fuel spent reaching the station?

What Kilomo should compare

• Break-even formula: (c/L saving × litres to fill) must exceed (detour km × consumption L/km × local price). At 8 L/100km, a 2 km detour burns just 0.16 L — easily beaten by saving 5 c/L on 40 litres.
• At 10 c/L saving on 40 litres: you earn €4. A 5 km detour at 8 L/100km costs ~€0.16 in extra fuel. The detour wins by €3.84 — before you even count your time.
• Partial fill-ups change the equation sharply: filling 20 litres at a 10 c/L gap earns only €2. A 5 km detour is borderline; 10 km is not worth it.
• Time cost is often the real constraint: a 10-minute detour that becomes 20 minutes in peak traffic is worth roughly €1–3 in opportunity cost — enough to turn a marginal detour into a clear skip.

The answer to "is it worth the detour?" is almost always a number, not a feeling. This guide gives you the formula, three worked examples across different scenarios, and the shortcuts that make the mental maths fast enough to do at 90 km/h on the motorway.

The three numbers you need

Every fuel detour decision reduces to three inputs:

1. How many litres you're filling — the larger the fill, the larger the gross saving
2. How much cheaper the other station is, in cents per litre — the raw price gap
3. How far out of your way the stop is, in kilometres — the detour cost

Time cost matters too — but fuel cost first, time second.

The formula

Gross saving = litres × price gap (€/L)

Detour fuel cost = detour km × your consumption (L/km) × price per litre

Net saving = gross saving − detour fuel cost

If net saving > 0, the detour pays off on fuel alone. Then you decide whether the time is worth it.

The consumption term catches people out. At 8 L/100km, a 5 km detour burns 0.4 L — at €1.70/L that's €0.68. Almost always negligible against a real price gap.

Three worked examples

Example 1 — The obvious yes

Scenario: A Metz frontalier filling up before crossing into Luxembourg.

• Filling: 50 L diesel
• French station: €1.78/L, Luxembourg station: €1.65/L → gap = 13 c/L
• Detour: 3 km off the direct A31 route
• Consumption: 7.5 L/100km

Gross saving: 50 × €0.13 = €6.50 Detour fuel cost: 3 × 0.075 × €1.65 = €0.37 Net saving: €6.13

Result: clear yes. Three minutes out of the way for over six euros.

Example 2 — The borderline

Scenario: A weekend driver passing near a cheaper station that adds a meaningful detour.

• Filling: 25 L petrol
• Gap: 10 c/L
• Detour: 8 km
• Consumption: 8 L/100km

Gross saving: 25 × €0.10 = €2.50 Detour fuel cost: 8 × 0.08 × €1.72 = €1.10 Net saving: €1.40

Result: marginal on fuel. Worth it in 5 minutes, not worth it in 15 minutes during rush hour. Add your time cost before deciding.

Example 3 — The clear no

Scenario: A driver with a nearly-full tank adding a small top-up at a station well off the route.

• Filling: 18 L
• Gap: 8 c/L
• Detour: 12 km
• Consumption: 8 L/100km

Gross saving: 18 × €0.08 = €1.44 Detour fuel cost: 12 × 0.08 × €1.70 = €1.63 Net saving: −€0.19

Result: you lose money on the fuel alone, before adding a minute of your time. Skip.

Adding time cost

For many drivers, time is the binding constraint. A practical rule:

A 10-minute detour worth €1.50 is irrelevant when you're saving €6. The same detour is wrong if you're saving €1.40.

The critical number is when extra time risk enters: peak-hour detours that show as "4 minutes" on a map can easily become 12–18 minutes in practice. When the fuel saving is marginal, the expected detour time, not the minimum, is what matters.

The fast mental shortcuts

When you can't do the full calculation:

Always worth it: detour under 3 km + filling 40+ litres + gap above 10 c/L. Net saving is almost certainly above €4.

Usually not worth it: detour over 10 km, or filling under 20 litres, or gap under 6 c/L. Do the maths before deciding.

Stop and check: you're under 30% of a tank and the price gap is small. It may be better to fill now at the less-good price and save the better stop for next time with a larger fill.

Full tank = bad detour logic: filling 8 litres because you happen to pass a cheap station gives you €0.80 at a 10 c/L gap. No detour is worth it for that.

How Kilomo applies this

Kilomo runs this calculation for every station on a route page. The "worth it" signal next to each station accounts for:

• The live price gap against your departure-country average
• The estimated detour distance from the direct route
• A conservative consumption assumption

It does not currently add time cost — that depends on your schedule. For commuters on regular routes, the fuel-only signal is usually accurate enough. For borderline detours in congested areas, add your own time penalty before committing to the stop.

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