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.
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DJ Phoenix (my modded) Hi-1200/240Tow/75FS/80DD/100/140SS +Minotaur
ASE Master Auto Technician

2001 Ford Excursion 7.3 4x4
Projects. 1970's Hustler and Max II Amphibious ATV's