Partial fuel fill
 

I have noted that that my daily milage improves towards the end of the tank. So I've started some semi-controlled testing.

Procedure
-Start with a full tank
-When the tank nears empty, partially fill it.
My tank is 16.5gal. I try to fill it to 10+ the reserve basing off what the SG2 says was used.
-Record trip milage from car and SG2, SG tank mpg, and gal filled
-Do not reset the car trip odometer
-Fillup the the scanguage
-Repeat for 2 to 3 weeks, and then do a total fill up so you can calculate the 'tank' average for all the partial fills

You end up with a table like this:

date.........trip odo(running total)..... SG odo.....SG mpg......fuel gal
...................308.8.......................... 306...........24.9........6.321
4/18.............520.2..........................209. ..........25.4.......10.000
4/22.............714.7..........................192. ..........22.9.......8.63*
4/25.............905.6..........................189. ..........24.7.......13.974
totals 905.6 896 38.925
'tank' mpg 23.26

Which isn't much better than Mandy's average, but* I replaced the front brakes and did a coolant flush in the middle of the test. That burned fuel and dropped the average.
Right now I'll do a total fill, before going back to a set of partial fills. Which will be at a level closer to half. I choose ten at the beginning because expecting to be visiting the station more often.

I found (I THINK) that I got better mpg using the first 4-5 gallons from a 19 gallon tank than with the full tank. 87 Nissan 3.0L.

I attributed this to the lighter vapors from fuel slosh with an almost full tank.

Curious but...do you think you could be getting better milege towards the end of your tank because your vehicle weighs less? I'm not the smartest guy on this board, far from it, but as an airline pilot, we use less fuel when the airplane weighs less. AvGas weights 6 lbs. per gallon. jet fuel is 6.7 lbs per pound. My guess is car gas is the same as AvGas....so if you are half full on a 20 gallon tank, then you are driving around about 60 lbs. lighter and may accound for a small increase of 1 or 2 MPG. Just my guess.

If you are getting your mileage reading with a scangauge then think about this . . . as your tank gets empty there are more fumes and increased fuel surface to evaporate the gas in the tank which gets vented to the engine and burned. However the Scangauge only measures the liquid fuel being injected so because there are gas fumes added to the intake the computer compensates and injects less gasoline thus burning less liquid fuel. You really end up with the same result when you fill up again unless the fumes end up burning more efficiently than the liquid it came from?

Also it doesn't take so long a period of running to heat up a couple of gallons of gas in a return to tank system. Since many stock fuel pumps are capable of supporting a couple of hundred HP and you're using maybe 20 hp, that's 90% of the fuel that comes up the line running through a warm engine bay and back to the tank again.

Here's the results of a regular full tank:
260.6 miles......11.832 gals.......22.02mpg
So there is a slight improvement for the partial fill so far.

I'm only using the scanguage to get an idea of what the milage is between total fill ups of the tank. The 23.26mpg is calculated from the truck's trip odometer and total gallons put into the tank. This is how I enter the gaslog entries.

I also notice better milage from the bottom half with the Prius. It is likely from the weight, but also be do to less column pressure in the tank. The pressure of a full tank, and expanded bladder in a Prius, could be pushing more fuel through open pumps and injectors. It also may add up to no effect on the system.

The scanguage was reset, so I'll be doing another regular fill before trying the partial again.

In the 70's there was a fellow named Tom Ogle who claimed to get 100 plus miles to the gallon in a Ford Galaxy 500. I never met Ogle, but knew several of the other people involved, and worked for Jim Peck when I was a teen.
https://www.linux-host.org/energie/etomogle.html
I also knew another of his silent partners (not the one who was killed). Apparently Ogle was very fond of taking in partners. One not mentioned was Bales Transmission .

More links here: https://rexresearch.com/ogle/1ogle.htm

Of the ones I have talked to, all claim that running a ICE on fuel vapor is legit.

Scangauge doesn't measure fuel injected. It measures air flow, and calculates fuel from that, based on the air/fuel ratio.

And I don't think any vapors are getting to the engine, since the fuel pump intake is at the lowest point of the tank.