When I was younger, one of the parents in the neighborhood was a bread delivery driver. I can still remember her ranting about an incident one snowy winter day. She couldn't drive as fast as all the other trucks on the highway and they were complaining to her over the radio. Her response was "I have no traction, I'm hauling a load of bread!"

A truck carrying bread is pretty close to the weight of an empty truck. The fuel efficiency of this truck is better than a truck hauling a load of flour. But... my guess is that per kilogram of cargo, hauling flour is much more fuel efficient than hauling bread and therefore it is better for the environment to haul flour around and then bake the bread as close to the source of consumption as possible.