its rotating mass and it makes a huge difference in mpg, the surface area of the tire can make a big difference too i mean look at race bicycles there is a reason why they have such skinny tires ,and wind resistance because he is now lower there is less air he has to move,
all this adds up in the end
Lol at comparing bike tires to truck tires. The race bikes tires are 50% of the width of other bike tires. Completely different when comparing to those.