Original tires with maybe 7,000 miles on them. Tire pressure checked regularly, and right before this trip. But wouldn’t any “tire circumference error” equally effect both the car’s display and calculated mileage? Sure, lots of variables can creep in. But again, if the amount of fuel actually added varied, one would expect errors in both directions tank-to-tank. It’s the consistency in the error in one direction that is suspicious, not that the car’s calculation isn’t perfect.