MPG improved with heavier tires???

Seems very unlikely that any tire could change mpg that much (particularly on a truck, where tires are not a great source of resistance compared to the aerodynamics and weight). I mean, my brother's Pruis lost a few mpg when he put different tires on it. I suspect there is another issue causing the calculations to be off.

I lost about 1 mpg freeway when I put LTs on my '11 F150.