For the illustrative 4-mile detour below, a 12-gallon fill-up breaks even at about 47 cents per gallon when 10 extra minutes are valued at $25/hour and the stated vehicle-cost assumptions are included; counting fuel alone, the threshold is about 4 cents per gallon. These are scenario inputs dated September 29, 2026, not live gas prices or measured national vehicle-cost averages.
A cheaper sign price is not the full trip cost. The useful question is how much must you save per gallon before extra miles, time, tolls, and vehicle costs are outweighed?
How do you calculate the break-even gas discount?
Break-even discount per gallon
(extra fuel + tolls + optional vehicle wear/depreciation + optional time value) ÷ gallons purchased
Use total additional miles for the full detour, not the one-way distance. Calculate extra fuel as additional miles ÷ your MPG × local price per gallon. Use your own observed MPG if you track it; the U.S. Department of Energy and EPA’s FuelEconomy.gov MPG guide explains the miles-driven ÷ gallons-used method. Add tolls only when the detour creates them. Vehicle wear, depreciation, and time are optional economic costs; leave them out if you only want to compare immediate pump spending.
What does a 4-mile detour cost in this example?
Assumptions for this worked example, as of September 29, 2026: 12 gallons purchased, 4 additional miles total, 27 MPG, gas at an assumed $3.50/gallon, maintenance and tire allowance of $0.10 per added mile, optional mileage-related depreciation of $0.15 per added mile, 10 extra minutes valued at $25/hour, and no additional toll. Every rate is an illustrative input to replace with your own numbers.
| Cost item | Calculation | Example cost |
|---|---|---|
| Extra fuel | 4 ÷ 27 × $3.50 | $0.52 |
| Maintenance/tire allowance | 4 × $0.10 | $0.40 |
| Optional depreciation | 4 × $0.15 | $0.60 |
| Optional time value | 10 ÷ 60 × $25 | $4.17 |
| Total economic cost | Sum of example items | $5.69 |
| Break-even discount | $5.69 ÷ 12 gallons | About $0.47/gallon |
If the alternate station is only 20 cents cheaper, 12 gallons save $2.40 at the pump, so this full-cost example does not break even. If you count only extra fuel, the 4-mile detour burns about 0.15 gallon, costing about $0.52; the fuel-only threshold is about 4 cents per gallon. The different thresholds reflect which costs you choose to count.
Should you count time and vehicle wear?
Time value is an opportunity-cost choice, not a bill. Run the calculation with and without it. Likewise, include only vehicle costs that actually change because of the added miles; do not automatically assign fixed monthly expenses to one trip.
What if the cheaper station is already on your route?
Count only the miles and minutes caused by choosing that station. If you would pass it anyway, the incremental detour may be close to zero. If you combine the stop with another errand, use only the extra distance and time attributable to the gas stop.
Frequently asked questions
Do I use one-way or round-trip detour distance?
Use the total additional distance for the entire trip, including the return or route back to your normal path. If a map says the station adds 2 miles each way, enter 4 additional miles.
Should I use the EPA-rated MPG or my real-world MPG?
For a personal cost estimate, use your observed MPG when available. FuelEconomy.gov describes calculating MPG from miles driven divided by gallons used; rated MPG is a comparison estimate, not a guarantee of your individual result.
How many gallons should I divide by?
Use the gallons you expect to buy on this stop. A 20-cent discount saves $1.60 on 8 gallons and $3.00 on 15 gallons, before any detour costs.
Sources and assumptions
- FuelEconomy.gov, “How to Calculate Your MPG” (U.S. DOE/EPA-administered site): source for the miles-driven divided by gallons-used MPG method.
- FuelEconomy.gov Trip Calculator (U.S. DOE/EPA-administered site): lets drivers customize vehicle MPG and fuel prices for trip-fuel estimates.
- DCLifeLab calculation, September 29, 2026: all gas price, wear, depreciation, time, distance, and fill-up amounts in the example are explicitly illustrative assumptions; the arithmetic is shown above and is not an official cost estimate.
For related calculations, see the Smart Living & Shopping guide, the commute cost calculator, and our analysis of Costco gas quality versus TOP TIER.