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December 26, 2007

Gassing up: when it's cold or hot?

A friend forwarded me an email the other day for my perusal. Someone suggested there was an advantage to filling up your car with gasoline in the morning (colder weather) compared to the afternoon (warmer weather). The principle behind this strategy is when the gas tanks below ground are cooler, say 32-deg-F at their coldest (probably warmer due to insulation in the ground), the fluid is more dense and you get more kilograms of fuel per unit of volume registered on the gas pump. Then, upon warming, the liquid expands and you get a greater volume of gas and go more miles.

The temperature dependence on density is a fact. But, what's the magnitude of the effect? My intuition led me to believe nil. So, I made a few assumptions and decided to figure out how much the magnitude of the effect was. I asked about 15 Ph.D. chemists (including myself) how to find the density of gasoline at 32-deg-F and no one knew. After I recovered from the embarrassment, I continued looking.

I Googled "buy gasoline during the coolest time of the day — early morning or late evening — while the gasoline is most dense", (this phrase was reported to be found in The New York Times, September 24, 2001) and stumbled on a flow meter tutorial. While it was published on a corporate site selling flow meters, it seemed convincing; pumps are fitted with temperature compensating flow meters. The tutorial is pretty interesting.

Another source stating essentially the same result is from Edmunds.

I'm convinced these flow meters do what they are supposed to do, but I'll keep hunting to find more support for this.

I'll also be hunting for the density of gasoline, 2,2,4-trimethylpentane (isooctane), or any other similar hydrocarbon at low temperature to see what the magnitude of the effect WOULD be (probably less than a cup of Starbucks).

Thanks for the question Tina.

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