with a 20 gallon tank and 10 gallon halfway mark you'll be running about what? 7lb/gal so 70 lbs difference from full or empty....maybe 1/10 mpg gain. nothing outside the normal variation.
as for filling up in the AM: that's logical but not thought out urban myth. gas is stored many feet underground and the temperature doesn't fluctuate much down there. ever been in a cave? the mouth is close to ambient temp but get a hundred feet in and it'll vary 5 degrees between a 90f day and a 0f winter night. even in the northern US permafrost only reaches a few feet down through the soil.
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"
I think the myth about filling at halfway vs. filling at E is all due to variations in gas tank designs.
My Saturn for instance APPEARS to go through gas after the halfway point VERY quickly. Take into consideration the design of the gas tank and you find that halfway isn't exactly 6.25 gallons, it's probably closer to 5 gallons or less.
In winter, in the freezing north - home of the climatically challenged, it's smart to keep the tank at least half full. You never know when or where an old car will decide to break down and it's really nice to have a lot of gas to burn if you need to wait for help to arrive.
More related to reliability then fuel economy I have always had at least half a tank simply to stop the pump sucking up the rubbish which settles in the bottom of the tank.
It also allows you the option to add fuel at your choice not that dictated by a flashing light on the gas gauge.
If you are about to run out then you pay whatever the gas station asks since you have little choice.
Also I like to fill at places having a high turnover since the fuel is normally sitting for less time than some place off the main traffic flow where the fuel may sit for ages before it is used and replaced.
By the way the differences in economy posted by PaleMelanesian amount to less then I MPG in 55 or between one and two percent.
The other part of the myth than spawned my question was the claim that extra airspace in the tank was a bad thing because there is too much room for vapor. Isnt the tank a closed system? So would one get better mileage on a full to half tank or half to empty tank?
I get slightly better mileage with the tank half full, but after I drive out of my way to stop an extra time or have to get gas at a higher priced station, it comes out the same. I ride without the front passenger seat most of the time to make up the weight difference; once it started getting cold I didn't want to pump gas in the cold cold wind any more than I had to.
Vapors aren't going anywhere but back into the liquid when it cools off; providing it got warm enough to evaporate to begin with.