Last few tanks I am refueling early morning as slow as possible. MPGs have increased dramatically! Theory: warmer temps. Faster fills. More fumes. Less gas.

 

Actually hotter fuel = larger volume, Most fuel pumps don't account for the volume difference, some states are mandating it.
Fill up in the morning when the fuel is cooler then it expands as the day gets warmer. The engine burns vapor it doesn't care about the liquid volume.

 

My fuel system guru says that because it’s a mixture that can include a lot of different stuff in highly variable proportions, the density of gasoline can vary over a range of as much as 15%. Now I wonder about EPA fuel economy tests. Surely they are conducted with a standardized, chemically identical gasoline mixture? Off to Google…

Nope. Well sort of. As always, it’s complicated. Here’s a random (ancient ancient but contemporary to the HHR) 2009 article.

https://www.caranddriver.com/feature...mpg-estimates/

 

There are pumps that compensate for variable volume. There was a consumer protection uproar a few years ago. People got upset at paying for a hot gallon of gas to find out it was really 3 quarts the next morning.
From: Gasoline pump - Wikipedia

 

I've always found that the best time is.........
Before you run out.

 

well, the tanks are buried so ground temp is pretty uniform if deep enough, its when that tanker delivers the fuel. you dont want that fresh hot fuel and the debris it stirs up as well... yall do know there is water in those buried tanks as well ( helps keep the tanks from floating up to the surface ) you want the fuel late night/early morning when its cooler , settled and denser. more bang for the bucks....

 

​​​​​​​if you’re saying that under certain conditions you might be paying for “foamy” fuel, I’ve often wondered about that. The retailers would probably love it.

Good point. Tanker trucks are not insulated. In the winter the fuel is likely to arrive cold and gradually warm up, albeit very slowly I am sure. In the summer the fuel is likely to arrive warmer than ground temperature and slowly cool off. If you’re going to pay that much attention then in the winter fuel soon after a new load of fuel arrives and in the summer fuel when the tanks are almost empty, although I don’t know how you know that.

or just drive slower with a light foot.

 

How does one burn through enough fuel in your gas tank fast enough to make this matter?

 

Never ever buy fuel while the tanker is unloading fuel. ALL underground tanks have some water in them because they breath outside air as fuel is dispensed. Air cools off at 60 degree (avg) and water condenses from humidity in the air.. Fuel is sold wholesale by NET gallons billed at 60 degrees winter or summer. Gross gallons is the total delivered but billed at 60 degrees. I was in the wholesale fuel business over 35 years. Just a FYI.

 

Feel the metal nozzle when fueling on a hot day. Cool? If not you need a different station. Should be noticeable.