Gas mileage tank to tank isn't important, it's the trend. I used to use my mileage to determine when a tune up (points, plugs, de-carb with a bottle of water down the carburetor) was needed (since 1967). Now I just use it to determine when the computer needs an adjustment (O2 sensor, ram air etc). For those who want to keep track of your trends and history, I offer the attached spreadsheet. Been using it for about 6 years on various vehicles.
Instructions: Enter Date, Gallons, and Cost. COPY cells (in green) Price/Gal, Miles/gal and Cost per mile. Calculates Price/gal (I added this for those times in my car when I really need gas, but don't want to pay the price I'm stuck with, just a couple of gallons to get me to work. Add the gallons and cost to both fills to get total). Calculates the MPG AND Cost per mile. This is interesting and may help to determine if that extra cost gas is really saving you money, or if the 87 octane is REALLY cheaper. I've added an Octane column to keep track of that too. Remember, the Octane data shows up on the next fill.
If anyone has any suggestions to make this work better, please let me know. As I said, I've been using it for about 6 years, so I am oblivious to any flaws. Not that there should BE any!
That's a handy piece of work! Look forward to meeting you folks at some point.
Glynnk