Originally Posted by Kontact

Actually, physics says it takes more energy to accelerate more weight, all else being equal.

Fuel mileage estimates include things like starting and stopping, and an average acceleration up to speed. But a cyclist in a flat area could only need to get up to speed once at the beginning of the ride, and physics will reward them for the momentum of their heavy bike by decreasing the minor decelerations from corning, wind, etc. Heavy bikes on flats are more efficient.

So you want to make blanket statements, but it doesn't sound like you've actually thought through the actual conditions mileage tests are performed under. I would expect your next myth buster is to tell cyclists it doesn't take more energy to ride faster since car MPGs are always lower for faster highway conditions. That wind resistance thing must be a myth!