get more gasoline per gallon when fueling up in cold temps?

Question

I read that you should fill your vehicle's fuel tank up on colder days as opposed to warmer days b/c on colder days the density of the gasoline is higher thus u get more gasoline (mass) per volume (gallons). That seems reasonable but how much of a difference would it really make? (eg how much more dense is it say at 10 deg. F as opposed to 60 deg F.) I tried to research this a bit and now I am also wondering what the difference is between 'winter grade' gasoline and 'summer grade' gasoline??

Answer 1

Okay, I know a bit about this. Actually, a lot.

Your specific question (density at 10F versus 60F) I don't have quite that data, but at 45F gasoline density is 46.270 pounds per cubic foot. At 60F = 45.850 pounds/cf. Extrapolating back to 10F would give 47.25 pounds/cf. An increase of mass of 3% (worth about 8 cents a gallon at $2.59/gallon).

Gasoline does definitely become more dense with colder temps and the effect is much more than with water.

As someone else posted, gasoline is (mostly) stored in underground tanks and the temperature there is very stable. I have, twice, gotten a noticeable hotter gasoline from the pump at a service VERY close to the refinery (2 miles) that had just taken delivery. The pump handle felt very warm (wondered if I had a fever at first or if the last guy's hand heat was still in it, but the longer I pumped, the hotter the handle got).

Ground temps do vary at a 4 to 11 foot depth that most of the tanks are at, particularly in snow country and at high elevations. Peak temperatures typically occur in October or so (there's a time lag compared to surface temps) and lowest temps are in May. But it only swings about 10F either way at most and those tanks see more effect from 2-3 loads of gasoline each week.

So yes, you get more mass of gasoline in winter due to colder gasoline being delivered and slightly colder ground temps.

All state/county weight&measures departments I've seen check gasoline pumps with a volumetric flask of sorts and the pumps are designed to accurately deliver a volume, not a constant mass. So you are paying for a gallon (hot or cold), just like the sign says. The station owner, however, pays for a "corrected volume", to equate it back to 60F. So 6,000 gallons of 90F gasoline at the end of the hot summer day costs the station a bit less than a 20F load in the winter. Those temps partially equilibrate with ground temperature, so customers rarely see that much of a swing in what they pump/buy.

Summer and winter gasoline aren't about density (directly), but about vapor pressure. Less important in fuel-injected cars, but the "Reid Vapor Pressure" in winter is formulated to be about 13 psi (from memory), whereas in summer, it is about 9 psi. In winter, a higher fraction of light-weight compounds are included (pentane, other C-5 stuff, etc) and less is used in summer. Otherwise, you'd have hard winter starting due to poor volatilization and/or more likely vapor lock in summer due to fuel boiling in the fuel lines of the hot engine compartment in summer. Those light compounds are a little less dense and contain a lesser heat value per volume, but it is a pretty small effect. Less than the temperature dependence of density.

The effect I notice more in winter is if I fill my tank the night before (40-50F fuel from an underground tank) and park outside on a -20F night, I can top off a bit more the next day and get a little more range on a tank. That effect could be as much as .7 gallons in a 16 gallon tank. But you have to be a total gearhead and geek to track it in that detail.

I've pondered, if I were to own a gas station, if I'd put in a heating system (gylcol-filled polyethylene tubing like in a radiant slab) beneath the tanks. 10,000 gallons of gasoline delivered at 60F could be "fluffed up" to 10,410 gallons at 115F. An extra $1000 in my pocket (400 gallons x $2.50/gallon). At a cost of about $80 of natural gas for the water heater in the back room.

But filling on cold days versus warm? The gasoline passes through the dispenser, meter, and hoses pretty quickly, so it doesn't make a big difference. A bigger difference would be the air temps and distance driven by the tank truck when they got their last delivery. And besides, if it is 60F and you're at a 1/4 tank, are you going to wait for a week because a cold front might move in?

Answer 2

It is highly likely that the type of gas is the least of you're issue for poorer milage. Think of how hard you're engine is working in the colder temps. It takes a bit longer for the colder oil to flow to all the moving parts in the engine in the winter as opposed to the summer. Thereby it has to overcome internal friction longer due to the slower moving colder oil. Aside from that, by nature we tend to let the vehicle warm up a little longer in the winter. Even a few unconscience mins makes a big difference in the milage you get over all. I am not saying that maybe the fuel they provide is slightly different in the winter, as that could be a possability. But that would have very little effect when you look at the whole picture overall about the milage you get winter vs. summer.

Answer 3

A study found that if you pump at a slow speed you are more likely to get more gasoline than if you pump at fast speeds.

No clue about the cold thing.

Answer 4

Many pumps nowadays compensate for this effect.

Winter/summer grade is a different deal, not density related but composition. I leave that point to others who know.

Answer 5

there's no difference in density, all storage tanks at the gas station are buried, which mean the gas stays a relative constant temperature summer or winter. as fore the grades, they up the oxygen content, supposedly to make it burn cleaner. hope this help you.