The freeway speed limit here is 80. Between 75 and 80, mileage often drops a lot because cars with overdrives will drop out of OD at that speed. You're running 800 to 1000 rpm faster on the tach.
Are you speaking specifically about Chevy trucks with the 5.3, or just trying to make a general statement? That certainly isn't the case with our Suburban with the 5.3; it holds OD and TC lockup easily at 80 mph, even on mild hills. It averages 15 mpg.
