Lots and lots..and….how bad can it be?
I just shut down early for the day I-80 west of Chicago about 50 miles or so.
Head wind of 25 mph I was per dash getting 4.3 mpg.. I was only limping along at 65mph..@80k gross.
Last trip WA to Buffalo, NY I got 6mpg with a mild tail wind.. The 6mpg was only acquired because I forced myself to run the entire trip slow. Yes I ran 67mph across SD , was difficult for me to do and it’ll never happen again 
389 mid-roof with dry van. Cummins 565hp with 3:55 axle ratio.. Set up for mountain driving with heavy loads..
Quarterly ifta reports show an average of 5.20 mpg. I only idle when I have to
