Fuel · the detour question
Is it worth driving further for cheaper gas?
Usually not. A cheaper pump has to beat the fuel you burn reaching it, and the gap people drive for is often smaller than the drive costs. A 15‑cent‑per‑gallon saving on a 12‑gallon fill is $1.80; six extra round‑trip miles in a 25 mpg car at $2.85 a gallon burns about 68 cents of fuel, leaving roughly $1.12 — before your time counts for anything.
The arithmetic
Two numbers, subtracted. Nothing else is needed.
Gross saving is the price difference per gallon times the gallons you will actually buy. Not the tank size — the gallons that go in.
The cost of getting there is the extra round-trip miles divided by your vehicle's mpg, times the cheaper pump's price. That quotient is the gallons the detour consumes; the price turns it into money. Use the cheaper price, because that is what you will pay to replace what you burned.
Net saving is the first minus the second. Below about $2 it is not worth arranging your route around, which is the threshold this site uses before it calls a detour worth considering.
Two worked examples
The one that does not pay
A pump 3 miles out of your way is 15 cents cheaper. You are putting in 12 gallons. Your car does 25 mpg and the cheaper pump is $2.85.
Gross saving: 0.15 × 12 = $1.80. The detour is 6 miles round trip, so 6 ÷ 25 = 0.24 gallons, and 0.24 × 2.85 = $0.68. Net: $1.12. Fifteen minutes of driving for a dollar and change.
The one that does
A pump 4 miles out of your way is 40 cents cheaper. You are filling 15 gallons, your car does 28 mpg, and the cheaper pump is $2.60.
Gross saving: 0.40 × 15 = $6.00. Detour: 8 ÷ 28 = 0.29 gallons, times 2.60 = $0.74. Net: $5.26. That one is worth a deliberate stop.
Why the gap matters less than the gallons
The saving scales with how much fuel you buy; the cost scales with how far you drive. A big price gap on a five-gallon top-up is worth less than a small gap on a full tank. This is why a detour that pays for a pickup rarely pays for a hatchback, and why the same pump is worth visiting on an empty tank and not on a half-full one.
It is also why a detour is usually worth it only when you were going that way anyway. Once the extra miles are zero the entire cost side disappears, and any price gap is pure saving.
What this leaves out
Your time, vehicle wear, traffic, and the chance the posted price is out of date. None of them favour the detour, so treat a borderline number as a no. Fuel prices also move faster than most price feeds, which is why every price here is shown with the time it was observed rather than presented as current.
Check live prices near you — the detour cost is applied to each station, so the comparison is already net of the drive.
Common questions
Is it worth driving further for cheaper gas?
Usually not, unless the price gap is large or the tank is big. Multiply the price difference per gallon by the gallons you will actually buy, then subtract the fuel burned on the extra round-trip miles. A 15-cent-per-gallon saving on a 12-gallon fill is $1.80, and six extra round-trip miles in a 25 mpg car at $2.85 a gallon costs about 68 cents of fuel — leaving roughly $1.12, before you count your time.
How do you calculate whether a gas detour pays off?
Gross saving is the price difference per gallon multiplied by the gallons purchased. The cost of the detour is the extra round-trip miles divided by your vehicle's mpg, multiplied by the cheaper pump price, because that is the fuel you burn getting there. Net saving is the first minus the second.
How much cheaper does gas have to be to be worth the drive?
It depends on the gallons and the distance, not on the price gap alone. For a 12-gallon fill and six extra round-trip miles at 25 mpg, the gap needs to be roughly 21 cents per gallon just to clear $2 of net saving. Double the fill-up and a smaller gap clears it; double the distance and a larger one is needed.
Does the detour cost depend on which pump price I use?
Use the cheaper pump's price for the detour fuel, since that is what you will be paying when you refill. The difference is small but it keeps the comparison honest rather than flattering the detour.
What is not included in this calculation?
Your time, vehicle wear, traffic, and the risk the posted price is stale. None of these favour the detour, so a borderline result should be read as a no.