[Remco]

I can get get mileage rolling downhill too, but going up the hill on the other side.... Not so much..

LOL! That's fore sure. You can't count on those things.
An Expedition rental I had went negative while rolling downhill. I guess the digits rolled over.
Either that, or it realized how stupid the number was and gave up.

 

[Remco]

The gas tank indicator is likely off by half an axle handle so these indicators mean very little:

The shape of the tank is not linear. The bottom and top range of whatever device they picked isn't linear.
There are conversion errors, temperature changes, expansion errors, errors due to sloshing of fluids, errors between cars, etc.

All these errors aren't insignificant and are accumulative.
Short term, they are for entertainment value only. It means absolutely nothing.
Long term, they are for reference only. They are an approximation at best.
It could be handy: If you see a drastic change over this long term number, you may have a problem. Maybe the gas you've switched to is not quite as good as they say, etc. That's all it is.

If you really want your actual MPG, divide however much fuel you added each time that's measured to a state calibrated precise standard, how many miles you traveled on your odometer. Reset odometer. Rinse/repeat. Simple. We've been doing that since Lassie was a pup because it works.

 

[Remco]

@pspaladi Exactly. The sticker MPG rating is under certain conditions, driven by a gramma at the speed limit, slow take offs, on her way to church without any traffic on the way, nor stop signs.

So the numbers generated by the computer are really is meant for reference. So they aren't accurate but could be useful:
If you have been seeing 26 mpg for a long time and it suddenly takes a nosedive, it is indicative something is going on.
For instance: If you lend your car to your teenage son and it suddenly drops to 10 mpg, he's probably street racing.