Cheap diesel vs Repsol: 0.3 litres and a feeling

Cheap diesel vs Repsol: a 0.3-litre difference and a feeling

A nearly empty tank of Bon Area diesel and a full one of Repsol. Same route, same average estimulante ilegal, same traffic. The result, according to the account that opens this comparison, is an average consumption of 5.5 litres per 100 km with the cheap fuel and 5.2 litres with the branded one. The difference is three tenths. What's striking is not the figure, but the driver's perception: at 2,400 rpm, with the cheap tank the car "seems to lack pull"; with the Repsol one, "it has power to spare". That's where the problem starts, because a feeling is not a measurement.

What a two-tank test actually measures

The test has a methodological flaw that anyone with basic statistics knowledge would spot in two minutes: a single measurement per fuel, no repetitions, no control of variables and no laboratory analysis. The 80-kilometre route mixes motorway and mountain roads, and was driven at night to ensure constant traffic. It's an honest attempt at control, but insufficient.

The most frequently repeated technical answer points out that the diesel arriving at a white-label service station and the one arriving at a Repsol come from the same CLH loading arm. What changes are the additives, and their effect on immediate performance is, at the very least, debatable. Some argue that the real difference lies in the condition of each station's tanks — water, sediment, fuel turnover — and not in the sign on the forecourt.

Additives: the 21st-century snake oil

The case for premium fuels rests on additives. They clean injectors, improve combustion, extend engine life. The argument is decades old and still lacks an independent study backing it with numbers. Against that, there's the accumulated experience of drivers who have covered 185,000 kilometres filling up exclusively at Bon Area without touching the injectors, or 300,000 kilometres on second-brand fuel with no incidents.

The counterargument also exists: a case of injector cleaning at 120,000 kilometres that the garage attributed to supermarket fuel, and which was resolved after 160,000 kilometres on branded fuel. That's a testimony, not proof. Correlation does not establish causation, and even less so with a single unit of analysis.

Fuel turnover matters more than the brand

The factor that does appear consistently in the analyses is each station's sales volume. A station with high turnover renews the fuel in its tanks frequently, which reduces the accumulation of water and sediment. A low-traffic branded station may have tanks in worse condition than a saturated Bon Area.

Real quality control doesn't depend on the logo. It depends on whether anyone checks the tanks. Some stations drain water weekly using reactive paste and pumps, while at others the staff just take your money.

The savings calculation: 1,200 euros over 240,000 kilometres

The economic question has an answer. For 240,000 kilometres at a consumption of 5 litres per 100 km, fuel spending ranges from 16,200 euros at 1.35 euros per litre to 17,400 euros at 1.45. The difference exceeds 1,200 euros. For a truck under 3.5 tonnes, the weekly saving reaches 15 euros with two tanks.

The full calculation, broken down by price scenarios, leaves an uncomfortable conclusion for premium fuel advocates: the extra cost is only justified if the additive produces a measurable fuel saving. And that, with a three-tenth difference in a test without repetition, is not proven.

What the feeling cannot prove

The effect of a full tank versus a nearly empty one on perceived power is a classic. Less weight, more agility. The difference between 5.5 and 5.2 litres could be explained by ambient temperature, tyre pressure or simply the order in which the runs were made. Nobody has measured actual power on a test bench with each fuel. Until that happens, the debate will remain where it started: in the driver's impression.