My 6 liter would start to drop off fuel mileage at about 75 mph. Around 80 (pulling trailer) it would be around 15 and at 90 it would drop to around 12. (But the snorting of the exhaust was pretty cool.) Chris’ 7.3 liter at 90 (open trailer) was still sitting around 18.
IIRC, when we trailered his 996 in a BIG enclosed trailer behind his 5.7 liter gas job it was in the 7 or 8mpg range (and you had to plan any vehicle passing two or three miles ahead. Stops were well planned, too! )