I am sure Jack and others will jump in here too, but MPG reports are like fishing stories!!! (especially when the user's name is FishChris)
So take it with a grain of salt. Tire rolling resistance will make a big difference. a highway tread Low rolling resistance tire could easily net +1 or more MPG's. Wind resistance is a big factor too. I'll see if I can find a graph to illustrate that and post it. Asphalt vs concrete highways will affect MPG too.