You'd find me totally hilarious then. I have multiple vehicles and each has a log book inside. EVERYTHING I do to the vehicle - add fuel, change a windshield wiper, change oil, etc. is logged with the mileage and date. Then I enter the fuel information into an Excel spreadsheet that calculates the exact fuel mileage and how much I've paid for fuel for that vehicle. When I say that my Toyota 4Runner gets 22.77 mpg overall you can bet that's correct for the life of the vehicle. Here's a screen shot of a portion of the Toyota's spreadsheet:
All that information may not mean much in the short term. But after several years (I tend to keep vehicles for many years) that history is invaluable - and it sure helps when I finally sell the vehicle. I don't choose my vehicles by fuel mileage either - but I love knowing exactly what the fuel mileage of each vehicle is.