Do I get more petrol at night?

September 21, 2005 3:55 AM

Is it true that I get more bang for the buck if I fill up my tank at night? The argument is that matter expands when it's heated; so during the night, I get more volume of petrol for the same price? I know the difference is miniscule, but is the argument true?
posted by arrowhead to travel & transportation (9 comments total)

It is far cheaper at night, I find. For example in Toronto: $1.04/L day, to $96.7/L night. As well, if you fill during the day, you loose some gas due to evaporation.
posted by strangelove at 4:08 AM on September 21, 2005

The matter is gas in the storage tank, thousands of gallons of it, deep underground. This matter will not change temperature due to the few degrees difference between daytime and nighttime temperatures.
posted by caddis at 4:16 AM on September 21, 2005

Your tank, however, is much more likely to heat up during the day.
posted by NinjaPirate at 5:11 AM on September 21, 2005

Gas pumps do some adjustment for that (read them, they even say something about Volume adjusted to X temperature on them), so unless the calibration on the pump is off and the adjustment is wrong, you'll just pay more for that denser gas.
posted by jacquilynne at 5:30 AM on September 21, 2005

Modern gas pumps do modify the volume pumped based on the temperature of the fluid. So, you can't win. But perhaps there is an old gas pump available.

Let's assume your gasoline is pure heptane ("assume a spherical cow..."). At 40C, you get .661 kg for every liter pumped. At 0C, you get .702 kg for every liter pumped. So you could get 6% more gasoline, if you pumped it at 0C/32F as opposed to 40C/104F. If your day and night temperatures are closer together than that, the difference will be less. If the storage tank doesn't fully heat or cool to match the outside temp, the difference will be less.
posted by jellicle at 5:51 AM on September 21, 2005

Just to amplify, gas volumes are normalized to 15 C in Canada, 60 F (15.56 C) in the US. This is the API standard temperature for measuring density in any petroluem, crude or product. So you get the same energy content/$ regardless of the time of year or day.

Also, unless you drive around with a rag in your fill-hole, evaporation is insignificant.

Finally, prices in my neck of the wood seem to follow many cycles: a daily one, as the original querant notes, high in morning rush-hour, down by 10:30, back up by 4-6, down again in the evening. Super-imposed on this is a two-week cycle of highs and lows. If you time it right, and catch the long cycle around here, you can end-up paying 5% to 10% less at the bottom of the cycle than the top. It's very unpredictable though.
posted by bonehead at 6:07 AM on September 21, 2005

NinjaPirate's point is a good aside. All other things being equal, you'll be able to get more gas into your tank in the daytime than the nighttime. It'll be warmer, and thus (very slightly) larger.

A side question: Do you get better gas mileage when your tank is nearly full (and you're driving around tens of pounds of gas) or when it's nearly empty (and you lose more gas to evaporation into the empty space of the tank).
posted by Plutor at 7:47 AM on September 21, 2005

Assuming generally stable gas prices, you'll usually get more "bang for the buck" if you fill up midweek rather than on the weekend. (If prices are rising or falling rapidly, of course, this will overshadow the few cents a gallon you might save by filling up Wednesday evening rather than on Saturday.)
posted by kindall at 8:36 AM on September 21, 2005

Just to amplify, gas volumes are normalized to 15 C in Canada, 60 F (15.56 C) in the US. This is the API standard temperature for measuring density in any petroluem, crude or product. So you get the same energy content/$ regardless of the time of year or day. - Bonehead

I thought that should be repeated. Modern gas pumps are very sophisicated. They measure what is dispensed to the mililitre.

Also, the gas is stored in huge tanks - underground. The temperature underground varies a lot less than the ambient air temperature.
posted by raedyn at 3:47 PM on September 28, 2005