Have I been doing it wrong all these years?

If the speedometer/odometer is off, which they usually are, even hand calculated numbers will be inaccurate. Manufacturers aren’t going to extend your warranty by an inch so it’s a pretty good guess which way it’ll be off. Our first Cummins Ram was 8% off. Taller tires brought it closer to reality. I tip my hat to the GM Dmax so no negative intended. I believe other circumstances contributed to the increased mileage.