Free Gas? Not Likely

The local TV station ran a “tip” tonight just before a commercial break in the 11 o’clock news, which I’ve paraphrased here:

Buy gas in the morning or late in the evening. Pumps dispense gasoline by volume, not by density, so you get more gasoline when the liquid is cooler.

This seemed a little bit preposterous to me, so I did a little figuring. According to the first reference I could find on Google, the volumetric coefficient of thermal expansion of gasoline (at 20 °C) is about 0.000950 °C-1. In other words, one cubic metre of gasoline expands 950 mL if you increase the temperature of the gasoline from 20 °C to 21 °C. Assume the expansion is fairly linear, or that the coefficient remains fairly constant, within 10 °C or so (Gateco Oil’s MSDS for unleaded regular says the boiling point is just under 30 °C).

A typical passenger car has a gas tank capable of holding around 15 gallons, or about 57 L. For the sake of argument, let’s say the temperature of gasoline stored in tanks 3-4 metres underground is subject to just as much fluctuation as the surface air temperature (a dubious assumption at best, but we’ll give the news crew the benefit of the doubt). Let’s be generous and say the average overnight low in the summer is about 10 °C (50 °F) and the average daytime high is about 30 °C (86 °F), a 20-degree swing. (We’ll ignore the fact that regular unleaded would be boiling at that temperature. Remember, we’re being generous here.)

In that situation, 20 °C * 0.000950 °C-1 * 57 L gives us a ΔV of about 1 L, or just over one-fourth of a U.S. gallon. At $3/gallon, that means we got ourselves about 90 cents of free gas by tanking up at the crack of dawn, or about enough gas to drive our typical passenger car eight miles down the road.

Under more realistic circumstances — say, a 10 °C temperature swing (which is still huge considering these tanks are three metres or more underground and that the temperature of the Earth remains essentially constant over a 24-hour period at any given depth below about one meter) — your typical car gets a whopping 45 cents of free gas. If you have a smaller tank than our “average” 15 gallons, the savings is even less.

The only possible way this “tip” could be useful is if the station stores their gasoline above ground and you have a gas tank the size of a KC-10.

Dave says this is a massive conspiracy theory to sell more Krispy Kreme doughnuts by encouraging people to fill up in the mornings. I can’t say that he’s wrong.

posted by Chris on 29 April 2006 at 0003 in car

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