You are not the only one; most posters on this subject don’t know how to fill the tank properly, calculate their mileage, and appreciate the factors that influence mileage.
…you have sufficiently cowed me into knowing that I have no idea what I am doing when it comes to estimating fuel economy.
I didn’t mean to do that. Tracking your fuel economy is useful, and in your case, fun. Just keep in mind that the figures you get are estimates and that speculation regarding cause and effect to changes in estimated fuel economy is only speculation.
A Fit, huh? 45+ mpg? Cool.
Even a modest tailwind could make a difference with a modestly powered car like a fit.