How big a drop in mpg from flapping bodywork ?
 

I drove just over 130 miles to my parents' place.
Once I arrived, I did a fill.
The nozzle clicked at 2.101 gallons, and gas poured out onto the side of the car.
Doing the math, I was surprised, but doubtful.
On the return trip back, It took 3.741 gallons.
Yup.
NO
WAY
NO WAY the car got over 60 mpg.
But what had me curious was that the pump didn't just click. It actually poured over the side of the car like the tank was full on the 2.101 fill.
It only took seconds to fill, and took me by surprise.

3.75 gallons would mean the car got only 34 mpg on the way back.
Stock, the car gets 42 mpg !
Puzzled, I looked at the car and found an area that I rigged up hurriedly. It apparently had been flapping the entire time.
I radiused the edge on it, BUT there was a pocket of air that forms directly behind.

How much of a drop in gas mileage can you get from something flapping around ?
( I immediately think of what kind of mileage trucks that have huge flags blowing in the wind get )

I would think 4 - 5 mpg even in the worst cases on a car getting 40 mpg.

Can a gas pump shut off on its' own and spill gas onto the car if there are still a couple of gallons to go till full ?

It will shut off if the tank doesn't vent properly, but IDK about the spillaging.

Maybe... if every panel was flapping. The next tank should tell a story.

I'm thinking it's a defective pump.
I filled up just before I left.
Prior to that, I drove less than 10 miles while doing a tuft test.
It said I used 1.24 gallons.

So less than 8 mpg ?
That sounds like a car I used to have.
But that was an old mopar.

Yes. Had it happen to me in a rental. Thought it was full when it overflowed, so I turned the car in. Received a bill in the mail showing they had to put another 3 gallons in at $10/gallon.

My old Nissan Almera would easily trap up to 7 liters (almost 2 gallons) of air in the tank if the cap was on the low side of the slightest incline.
Typically the air acted like a spring and could make some fuel flow out of the filler neck once you stop filling.

A couple of gentle nudges and several more liters would go in.