Question of the Day - Is Cold Gas Denser? Part 2

I was asked if motorists should fill their gas tanks early in the morning when the temperatures are cooler. The colder gas will be denser and you will get more for your money. My reply indicated that the volume of the gas doesn't change much due to temperatures. Apparently I was wrong...

In fact, one reader wrote that, "jet pilots always calculate their fuel requirements in pounds and must convert to gallons when ordering the fuel. When ordering a large amount, temperature is factored in when doing this conversion. A gallon of jet fuel at room temp weighs about 6.7 lbs/gal. Reduce the temperature and the weight will increase to 6.8 lbs/gal."

Another reader sent a link to a flow meter for gasoline which discusses this subject under the subheading 3.2. One startling point raised on this Web site was this statement: "Even the New York Times offered advice to the consumer on this one with '... buy gasoline during the coolest time of the day — early morning or late evening — while the gasoline is most dense...' (New York Times, September 24, 2001, Empowered II Smart Energy Management, A clean car is an efficient car, page 7)."

So, there does seem to be variation in the volume of gasoline as temperatures change. The more significant question for motorists is, how much variation is there? And, can I take advantage of this to save money? The answer is almost certainly no.

First of all, if there really was an advantage to buying gas early in the morning, the oil companies would find a way to adjust for this variation. It's like going to a casino and thinking that you have discovered a strategy to beat the house. Don't you think that millions of people have already thought of everything possible? And if someone did figure out a way to win consistently (card counting) the casinos would change the rules (multiple decks of cards). Secondly, gasoline at most filling stations is stored in underground tanks where the temperature doesn't change more than a single degree overnight. How much will this temperature save? And finally, consumers looking for a way to reduce gas costs are much better off changing their own driving habits rather than trying to exploit an angle. Eliminating needless jack rabbit starts, cruising at the speed limit and coasting to a stop will save a measurable amount of gas. Filling your tank early in the morning is little more than an inconvenience, despite what the New York Times says.

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July 10, 2006, 9:24 AM

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