- Cold fuel is slightly denser; temperature changes affect energy per liter.
- Underground tanks stay thermally stable, with only 2–3°C variation daily.
- Pipes and pumps hold minimal fuel; no significant heating before pumping.
- Overall, morning or night fueling yields negligible difference in volume.
Fuel prices have hit record highs this year and the outlook is not encouraging. For millions of drivers who rely on their cars daily, fuel costs are a significant burden on the family budget. Naturally, many motorists have started seeking creative ways to cut these costs, whether through economical driving techniques or by choosing the optimal fueling moment. One of the most popular theories circulating online claims that fueling early in the morning or at night allows you to get more fuel for the same amount. But is it true?
Scientific Basis: The Thermal Expansion Principle
The theory behind this mit rests on a real and well-documented physical principle: the thermal expansion of liquids. Fuels, like any liquid, expand when temperature rises and contract when it falls. In practice, at higher temperatures, the same mass of gasoline or diesel occupies a larger volume, but the effective energy density is lower.
To understand better, let’s take a concrete example: one liter of gasoline measured at 15°C contains approximately 737 grams of fuel. At 25°C, the same liter will have a lower density, containing fewer grams of actual fuel. The difference is small, but it exists — about 1% for every 10 degrees Celsius.
Thus, the theory seems logical: if you fuel when the temperature is lower, you would obtain the same volume (you pay for liters) but with a higher density, hence more energy in the tank.
Reality at Modern Fuel Stations
Although the physical principle is correct, there is a crucial factor that most proponents of this theory completely ignore: how fuel is stored at modern stations.
In Romania, as in most developed countries, fuel stations use massive underground tanks, located at depths of 2-4 meters below ground level. These tanks are:
- surrounded by a thick layer of soil and concrete
- thermally insulated from ambient air temperature fluctuations
- kept at a relatively constant temperature year-round
- protected from direct solar radiation
Soil temperature at the depth of these tanks varies very little throughout the day — practically, it is almost constant. While air temperature can vary by 15-20°C between night and the hottest day, the fuel temperature in underground tanks remains within parameters, with variations of a maximum of 2-3°C.
What Happens Between the Tank and the Pump?
Some might argue that even though the fuel in the tank is at a stable temperature, it passes through pipes and pump components that could be affected by ambient temperature. Let’s analyze this aspect as well:
The Pumping System
- The fuel is pumped from the underground tank directly into the nozzle
- The transit time through pipes is a few seconds
- The volume of fuel in pipes and the pump is minimal (under 5 liters)
- In a busy station, fuel never sits in the pump long enough to heat up significantly
Economic Calculation
Let’s assume an optimistic scenario for the morning fueling theory:
- Temperature difference between night and the warmest day: 20°C
- Thermal expansion coefficient for gasoline: ~0.001 per °C
- At 50 liters, the theoretical maximum difference: about 1 liter
However, given that:
- Temperature in underground tanks varies by a maximum 2-3°C
- Only the fuel in pipes and the pump (under 5 liters) could be affected by ambient air temperature
- The exposure time is minimal
The real volume difference is negligible — probably under 50 milliliters for a full 50-liter fill. At current gasoline prices, this translates to a “savings” of a few dollars, insufficient to justify the effort of fueling exclusively in the morning.
International Standardization
In the oil industry, there are clear international standards for selling fuels. In most countries, including Romania, fuel volume is standardized to a reference temperature (usually 15°C). Modern pumps are equipped with automatic temperature compensators that adjust the displayed volume according to the real temperature of the fuel, effectively eliminating any theoretical advantage.
These compensation systems are mandatory in many jurisdictions and are calibrated and periodically checked by metrology authorities to ensure measurement accuracy.
Real Methods to Save Fuel
Rather than relying on myths, here are scientifically proven methods that can help you effectively reduce fuel consumption:
Driving Style
- Smooth accelerations and anticipation - avoid abrupt accelerations and braking
- Maintaining a steady speed - use cruise control on highways
- Reducing unnecessary load - remove heavy, unused items from the car
- Avoiding prolonged idling - turn off the engine if you’re stopped for more than 30 seconds
Technical Maintenance
- Correct tire pressure - underinflated tires can increase consumption by up to 3%
- Regular maintenance - dirty air filters, worn spark plugs, and old oil increase consumption
- Checking the fuel injection system - dirty or faulty injectors affect efficiency
- Wheel geometry - improper alignment increases rolling resistance
Planning
- Route optimization - reduce the number of separate trips
- Avoid peak hours - stop-and-go traffic consumes significantly more fuel
- Using traffic apps - choose more efficient routes
Conclusion
Although the idea of fueling in the morning appears physically logical, in practice it offers no real benefits. Modern fuel storage and distribution systems, together with temperature standardization and compensation, mean that the time of day you fuel has little impact on the actual energy you receive.
Instead, focus on factors that genuinely make a difference: driving style, proper vehicle maintenance, and smart trip planning. These can reduce consumption by 10-30%, a truly meaningful saving compared to the minimal theoretical gains of morning fueling.
Thus, we can classify this idea as part of the “popular automotive myths” category — it has a scientific basis, but practical applicability is almost non-existent in real-world modern fuel stations.
