Bottom lines...
Using gas gauge for measuring the difference in gas mileage between 14 and 16 mpg is moot.
Relying on gas station shut-off valves for measuring the difference in gas mileage between 14 and 16 mpg is moot.
I've been following this and earlier thread out of curiosity, but man... Mike, you are misguided.
I've been tracking my gas mileage by full tanks on long trips, and yet it varied between 11 and 19 mpg. That 8 mpg difference was recorded on the same stretch of a road, familiar for Utah folks (I-70 between Green River and I-15), driven at the same speed, in the same direction. The difference: head wind or lack thereof.
True, I know it's not exact, and admit it might be an anomoly but like I said, I notice it every time - about 100 miles on 1/4 tank. And 1/4 tank is +/- 6 gallons (which corresponds almost to 6 gallons per 1/4 tank for a capacity of 23 total - again, not exact, but close) To remove the gauge out of the equation I filled up at 1/4 tank and took 6 gallons. Even pump discrepency would only be about half a gallon.
I'm still driving and will test it again. After that I'll drive it until empty, put in 6 gallons, drive it until empty and see how far it goes. If it gets 100 miles in 6 gallons on a full tank, but far less on the same 6 gallons on an empty tank we'll have a problem, but no solution still.
Combined with the abnormal tank pressure it's probably not -the- problem but it seems to be contributing.
If I can replicate 100 miles in 6 gallons repeatedly, and then less once below half a tank (to remove the gauge I'll measure miles and gallons filled, and ignore the gauge) then there's something there....but might not be. Lot more driving to do.
On a full tank, at 19 gallons, I've pulled repeated 11/12 every time. Never higher. City driving only in winter with long idles in the morniing I get 8. I'm just not understanding why 6 gallons in a 23 gallon tank nets 100 miles. But overall tank (20 gallons since I never run dry) only nets 234/5.
I've now identified a potential cat problem, so maybe that's it...but I'm down to these two things....unless there are valve problems - but have a vacuum gauge coming (old one broke) to do a real motor test.