https://www.randakksblog.com/fuel-mileage-issues/"Back in 1976, I was in graduate school and did a research paper on automobile fuel mileage for a statistics class. The general thrust of the paper was to analyze many, many variables and determine what, if any, correlations existed between those variables and EPA official fuel mileage ratings. I considered a long list of obvious “mechanical” things like: engine displacement, number of cylinders, compression ratio, valve lift, cam timing, type of fuel system (carb vs. EFI), octane requirement, type of transmission, final drive ratio, length, height, width, wheelbase, overall “wet” weight, etc.
I also looked at “non-intuitive” variables like vehicle price, length of warranty, number of doors, country of origin, etc.
I won’t bore you with the details (statistics is like that!),
but the only thing that mattered (statistically) in my study was vehicle weight. I’m not saying this was the definitive analysis on the matter …far from it. After all, my beer budget was rather large in those days! But, it proved to me that basic physics always prevail."