Save on gas with morning fill-ups? Don’t bet on it

Some people say it’s better to buy your gasoline first thing in the morning, rather than in the heat of the day. That’s because gasoline, like all liquids, expands when heated. So, according to this advice, gasoline will be denser in the cool of early morning, meaning you’ll get more energy per gallon than later in the day.

The basic facts are correct, but the advice is not. Gasoline does expand and contract a little depending on its temperature. When gasoline rises from 60 to 75 degrees 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. The result is that the temperature of the gasoline coming out of the fuel nozzle varies very little, if at all, during any 24-hour stretch at any particular station.

Craig Eerkes, former chairman of the Petroleum Marketers Association of America, a trade group for filling stations, says that the expansion and contraction of gasoline due to day-long temperature shifts is, for the consumer, "Just so, so minuscule as to be almost nonexistent."

Judy Dugan, 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 also notes that today’s double-welled tanks tend to keep the gasoline at the same temperature at which it’s deliver 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 we did. We performed some temperature testing at our auto-test facility in East Haddam, Connecticut, where we have an underground fuel tank similar to a typical filling station’s. Over a few summer days we measured the temperature of each gallon leaving our dispenser nozzle both in the early morning (8:30 a.m.) and early afternoon (12:30 to 1:00 p.m.). 

Results. While the air temperature between filling varied by up to 12 degrees, the fuel in our underground tank stayed at a steady 62 degrees F. As a result, we found that after the first few gallons were pumped, the fuel temperature coming out of the nozzle varied very little between morning and afternoon.

At both morning and afternoon fill-ups, however, the first few gallons out of the nozzle were notably warmer than following gallons. The temperature between the first and tenth gallons, for example, dropped by between 8 and 17 degrees. This was a result of the gas sitting in the pump dispenser, which was warmed by 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 where the fuel hasn’t sat 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 points out, 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 five in the morning or two in the afternoon.

Bottom line. Even with the temperature swings we saw in the first few gallons pumped at our facility, we didn’t see a big penalty for the consumer. A 15-degree difference, for example, would result in a one-percent gain in volume. Or, just a few cents difference on 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.

—Gordon Hard

For more information on saving fuel see our reports on how to get the best gas mileage and where to find the cheapest gas. 

 

Comments

[George Morris]

One thing that you forgot to take into account is the temperature of gasoline in a consumer's car. During the hottest times of the day, gasoline within the car is more likely to evaporate, and those fuel vapors will escape when the gas cap is removed.

The best time of day to fill up is in the morning, especially on cool, humid days when the air is less likely to suck away fuel. By reducing the amount of fuel vapor in the air, savvy consumers can also reduce air pollution and protect themselves from lung damage.

[Randy Waltz]

Gordon I read your article with some fascination. What you wrote is very true. But the real story I think you are missing is, that cool gas in the tank is more gas when the auto heats up, not the tank at the station. I have seen this happen several times over the years. Fill a tank too full and let it set in the heat of the day and it will always overflow. Why because as you said in your article the Gas expands. So at a 16 gallon fill upe you may end up with .16 of a gallon more at the end of the day or mid afternoon than you put in the car, at the temperatures your article quoted. Make the temp variance even more and it will expand even more. I worked in the liquid fuels industry for a long time; Hydrocarbons, and they act the same way. On a hot day you better have the cool gas vented as it warms or the pressure is gonna build.

Randy

[Schoolgirl]

I have a question, and this is probably very stupid, but if you fill up in the morning and supposedly get more volume, what happens when the gas expands in your gas tank when you're driving around town and it's 100 degrees outside??? If your tank holds 12 gallons, why try to put in more on a cold morning?

[Gary Garnier]

Don't some (maybe most) newer pumps actually meter the gasoline flow on a mass basis rather than a volume basis? That would take any density variations from temperature differences out of the picture as far as what you get for your money

[MBrook]

And what about someplace like Arizona? The temp can change 30 to 40 degrees between the morning and afternoon. I started looking into this because I've been filling up on my way home at 5:00 and have been getting about 280 miles per tank. For no reason I filled up in the morning last week and I'm at 330 miles right now, and will get another 25 before I fill up tomorrow morning. No, I don't think the time of day would account for a 70 mile difference, but I also don't know what it would be. But that's why I'm checking things like this out.

[Paul]

Cold Gasoline is better than hot gasoline. A cold mixture of Air & Fuel tends to make your vehicle run more proficient or "better" unless you're driving a diesel in which case cold fuel is your enemy.
My .02.

[paul]

One item missing in this discussion is the fact that variations in air temperature will take hours to affect the temperature in the underground tank. A graph of temperature fluctuations in the tank would show that the gas inside reaches its peak at a different time than the air temperature peak. In fact, if the lag were 12 hours, you'd be buying gas in the morning when the tank temperature was at its highest.

[Ace]

I believe that the entire discussion is way off track. Pumped gas is probably measured dynamically rather than by volume in gallons. What this means is that at a colder temperature, denser gas registers higher dynamic pressure and so less of it by volume is pumped. When warmer, more volume is pumped to compensate for less dynamic pressure. The outcome in terms of energy per dollar spent will be the same. You could get more gas in the morning but you will spend more on it. Perhaps a scientific explanation from a petroleum engineer is in order.

[Eric]

Ace that is not correct. Just about all pumps measure gas by volume. Most even have disclaimers stating such.