Sudden Jump in MPG? What?

[online170]

The way you are measuring could account for it.

The only way to measure is to use GPS or Google to figure out EXACTLY the distance you drove. And measure volume from Fillup-to-Fillup. Our tanks are not square, our gas gage and odometer arent that accurate. Garbage in garbage out. Remeasure with your 5 gallon tank and see what you get.

That being said, my buick 350 2bbl in 71 skylark and 2.73 gears averaged me 20 mpg with mixed driving, so its not impossible. Another time and different car, my mechanical advance got stuck on a long highway trip and my mpg jumped 7 points on that trip. Guess it was the right setting for highway cruising.

 

[Clarmo]

10MPG??? I know exactly was it wrong!!!
You have a hole in your gas tank!
I can get 16-18mpg with 500HP big block and an 850 double pump holley.

 

[MrSony]

From my neighbor's house to a restaurant up the street is exactly 1 mile. Did it 5 times to check. Drove my car twice, went back and forth 2 times, so 6.4 miles accumulated. I would not rule out a hole(s) in the tank or lines somewhere, they are original. Will continue to report on my findings as I go about them, mods please to not lock due to long spaces of inactivity.