Two filling stations an hour apart on the same highway routinely post noticeably different prices for identical fuel. The variation comes from tax, logistics and local competition rather than from product quality.
Tax varies by jurisdiction
Fuel is taxed at national, regional and sometimes municipal level, and crossing a border can change the tax component substantially within a few kilometres.
Drivers on long routes frequently notice a step change in prices at a state or provincial line, which reflects the tax schedule rather than anything about the stations.
Because the tax is levied per unit of volume in many systems, its effect on the pump price is stable and does not move with wholesale prices.
Distance from the terminal is a real cost
Fuel is distributed by road tanker from storage terminals near ports, refineries or pipeline heads, and each kilometre of delivery adds cost.
A station far from any terminal pays more for the same product before it sells a single litre, and remote areas compound this with smaller, less frequent deliveries.
Island and mountain locations show the effect most clearly, since both add difficulty as well as distance to every delivery.
Competition at the site is decisive
A station with a competitor visible across the road prices very differently from one that is the only option for the next hundred kilometres.
Drivers cannot easily reverse course once past an exit, so an isolated site faces demand that is close to unresponsive to price.
Clusters of stations at major junctions therefore tend to be cheap, and the long empty stretches between them tend to be expensive.
Motorway service areas are a different business
Service area operators pay for concessions on the motorway network, and those agreements carry obligations such as continuous opening and staffing.
Fuel at those sites is also sold alongside food and retail, and the pricing of the whole site is planned together rather than fuel by itself.
Stations a short distance off the motorway serve local traffic as well and are priced for that market, which is why a brief detour so often pays.
Practical consequences for a long drive
Filling in towns rather than at isolated highway sites is the single change that makes the most difference over a long route.
Crossing a border with a full tank when moving into a higher-tax jurisdiction is worth planning, and the reverse is worth avoiding.
Running a tank low in remote regions is the expensive mistake, because it removes the ability to choose where to buy and hands pricing power to whichever station appears first.