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Did you use a tape measure and measure to compare the actual circumference of each tire?
The new tires are taller, giving greater circumference and therefore greater distance traveled per revolution.
Also, a new tire has thick tread depth, but an old tire has thin tread depth, further increasing the difference in diameters/circumferences/distances traveled per revolution.
OTOH, an old tire with thin tread may have less squirm and rolling resistance than a new one, or being smoother, less aero drag.
Could be, the difference in mileage is attributable to more variables than just differences in nominal height, or that once the new tires wear down, they won't be as fuel efficient.
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