I keep an Excel spreadsheet of MPG myself. I know it'* not a Bonneville guys, but here'* some comparison, Honda 1.5L. The gray boxes contain formulas that calculate everything as I enter the # of litres and # of km travelled. I don't let my tank run lower than 1/4, so the km you see is NOT indicative of total range on a full tank.
It'* pretty silly how much crappy mileage I was getting early in the year, most likely due to inclement cold Canadian weather. The lowest 22.8mpg number is after sitting in traffic for almost 3 hours in a snowstorm.
I REALLY REALLY do not understand the two most recent tanks, either.