Does Filling Your Tank During the Heat of the Day Waste Money?
It’s a popular belief that cooler gas gives you more for your money. Here is what actually happens when you pump fuel in hot weather.
You may have heard that it’s better to buy gasoline first thing in the morning rather than in the heat of the day. That’s because mornings tend to be cooler, and, like all liquids, gasoline expands when heated. It sounds logical—but is it true or merely an urban legend?
With a number of temperature records broken in 2026, such as Atlantic City, N.J., hitting an all-time high of 106°F, and Salt Lake City, Utah, setting a record of 109°F, this topic is sure to come up.
We conducted our own tests and interviewed experts, and found that although the basic facts are correct, the advice is not.
Does Gas Density Change with Temperature?
Gasoline expands and contracts slightly with temperature changes. When it rises from 60°F to 75°F, for instance, it increases in volume by 1 percent while the energy content remains the same. But filling stations typically store their gasoline in underground tanks, where the temperature variation during the day is much less than in the air above.
Are Gas Station Fuel Tanks Underground Temperature Controlled?
Rob Underwood, president and COO of Energy Marketers of America, a trade group that represents filling stations, says that “gasoline pumps dispense a full gallon irrespective of the temperature at the time.” He also says gasoline dispensers are checked in each state to ensure consumers receive a full gallon. “The National Conference of Weights and Measures performed an exhaustive four-year study to determine whether any changes to current procedures were necessary to compensate for temperature and determined that no changes were necessary.”
Judy Dugan, former research director for the California advocacy group Consumer Watchdog, says, “the temperature variation between day and night at an individual gas station is apt to be negligible.” She adds that today’s tanks tend to keep the gasoline at the same temperature it’s delivered for a while. “If fuel is warm when it’s delivered to a station, it’ll still be warm when it’s sold a few hours later.”
What Were the Results of Consumer Reports' Tests?
While the air temperature between fillings varied by as much as 12°F, the fuel in our underground tank remained a steady 62°F. As a result, we found that after the first few gallons were pumped the fuel temperature exiting the nozzle varied very little between morning and afternoon.
At both morning and afternoon fill-ups, however, the first few gallons from the nozzle were noticeably warmer than the subsequent gallons. The temperature between the first and 10th gallons, for example, dropped by between 8°F and 17°F. This was a result of the gas in the pump dispenser warming in the sun. At our underground tank, which stores premium fuel, it’s not unusual for the gasoline to sit for hours or even days between fill-ups–unlike a typical filling-station tank, which may be replenished every day or even more often. After pumping a car tankful of gas, 20 gallons or so, the temperature had declined to that of the underground tank.
For consumers, this indicates that you could be marginally better off getting gas when the fuel hasn’t been sitting in a sun-warmed pump assembly for very long, regardless of the time of day. But that only holds true if the underground tank is keeping the fuel cold. As Dugan says, that’s often not the case. Today’s double-walled tanks work just as well at keeping fuel warm as keeping it cool. If fuel is warm when it’s delivered to a station, it’ll still be warm when it’s sold a few hours later, whether that’s 5 in the morning or 2 in the afternoon.
Even with the temperature swings we saw in the first few gallons pumped at our facility, we didn’t see a big penalty for consumers. A 15-degree difference, for example, would result in a 1 percent gain in volume. That’s just a few cents’ difference in the first gallons pumped–not enough to change your schedule or routine in chasing costs, especially if it might increase your fuel consumption in the pursuit.
What Are Real Ways to Save Money on Gas?
Now is as good a time as ever to fine-tune your driving to maximize your fuel economy and ease the squeeze on your wallet.
Consumer Reports experts pulled together these proven tips to help you save money. Click through to read the full description of each tip.
1. Obey the Speed Limit for Safety and Efficiency
2. Drive Smoothly
3. Skip Premium Gas—if You Can
4. Pay Attention to Aerodynamics
5. Turn off the AC when it isn’t hot
6. Check Gas Prices Online
7. Minimize Travel to Save Gas
8. Buy Good-Quality Gas
9. Stay at Half a Tank
10. Check Tire Pressure
