Is a detour for cheaper fuel worth it? Calculate the break-even point
Compare pump savings with the extra fuel used on a detour, using worked US and metric examples and clearly stated assumptions.
By Bell Park Products LLC · Updated November 4, 2026
A lower pump price does not always mean a cheaper fill. A detour uses fuel, and its cost can consume a small saving before you consider the extra time. Compare the same useful purchase at both stations and subtract the cost of the additional driving.
This is a fuel-only comparison. It does not place a value on your time or include tolls, vehicle wear or a membership fee. All prices below are hypothetical, not a report of current local prices. Use observed prices for the correct fuel grade and a realistic economy figure for your own calculation.
Write down four inputs
You need the number of gallons or litres you would buy anyway, the difference between station prices, the additional driving distance and the car's fuel consumption for that driving. “Additional” means compared with your convenient route. A station four miles away is not necessarily an eight-mile detour if it is already on the way to your destination.
Start with gross pump savings: purchase volume × price difference. Then estimate detour fuel cost with the fuel cost calculator. Use the extra distance rather than the entire journey. If you enter the full extra out-and-back distance, leave the return-journey option off so you do not double it again.
A worked example in US gallons
Suppose the convenient station charges $3.60 per US gallon and an alternative charges $3.45. You plan to buy 12 US gallons. The alternative adds 8 miles of driving, and you estimate 30 US MPG on that detour.
The gross saving is 12 × ($3.60 − $3.45) = $1.80. The detour uses 8 ÷ 30 = 0.2667 US gallons. Valuing that fuel conservatively at the convenient station's $3.60 price gives a detour fuel cost of $0.96. The estimated net fuel saving is therefore $1.80 − $0.96 = $0.84.
| Calculation | Example result |
|---|---|
| Price difference | $0.15 per US gallon |
| Gross saving on 12 gallons | $1.80 |
| Extra fuel for 8 miles at 30 MPG | 0.2667 US gallons |
| Extra fuel valued at $3.60/gallon | $0.96 |
| Estimated net fuel saving | $0.84 |
This calculation values consumed fuel; it is not necessarily the exact difference between today's receipts. That depends on whether you buy the same volume or refill to the same final tank level. Keep the comparison consistent and state which price you use to value the detour fuel.
Find the break-even purchase volume
Divide the extra fuel cost by the per-unit price saving. In this example, $0.96 ÷ $0.15 = 6.4 US gallons. Buying more than 6.4 gallons produces a positive fuel-only saving under the stated assumptions. Buying 5 gallons saves $0.75 at the pump but incurs $0.96 of additional fuel, producing a $0.21 loss.
You can also solve for the largest extra distance. Multiply the gross pump saving by MPG, then divide by the price used to value consumed fuel: $1.80 × 30 ÷ $3.60 = 15 miles. At exactly 15 additional miles, the modeled pump saving and fuel cost are equal. An existing errand that reduces the detour from 8 miles to 2 can change the comparison substantially.
The same calculation in litres
Suppose you would buy 40 litres, save 0.06 currency units per litre, add 10 kilometres and consume 7.5 L/100 km. Gross savings are 40 × 0.06 = 2.40. Extra fuel use is 10 × 7.5 ÷ 100 = 0.75 litres.
If you value the fuel at 1.60 per litre, the detour costs 1.20. Net fuel savings are 1.20, and the break-even purchase is 1.20 ÷ 0.06 = 20 litres. No currency conversion is needed if both stations use the same currency; if they do not, convert their prices using an appropriate exchange rate before comparing.
Test the inputs that can change the answer
A queue, cold start or congested side road can make highway MPG a poor detour assumption. The US Department of Energy and EPA's driving-efficiency guidance explains that driving conditions and idling affect fuel use. Rather than borrowing a universal percentage, repeat the example with a less efficient figure relevant to your car.
At 20 MPG instead of 30, the same 8-mile detour uses 0.4 gallons. At $3.60 per gallon that costs $1.44, leaving just $0.36 in estimated savings on the 12-gallon purchase. Add a toll or membership cost only when it is genuinely additional to this decision, and keep it separate from the fuel estimate.
Verify price freshness, fuel grade, eligibility for discounts and the extra distance before leaving. Do not buy more fuel merely to make the break-even math look attractive. For unit checks, see US versus Imperial MPG. For a better economy input, use the measured fuel economy guide. The useful result is a transparent comparison based on your journey.