Back in 1987/88, I worked at a service station.
Holden fitted a flat 80L tank under the Commodore wagon that couldn't be filled on a flat surface more than about 65L.
We had ONE pump with a sloping section of concrete that allowed the Commodore to actually fill if they drove through from left to right rather than right to left.
Had irate customers waving manuals in my face, refusing to pay, calling scam etc....advised them to call consumer affairs, and they'd come, test the pumps, attach their tags.
Taught me that when tracking mileage, same pump, same station every time.