Hi there, the 78 miles per hour caught my eye. My guess is that this speed would yield less than 20 mpg with the non hybrid and maybe 21-22 with a hybrid. It is just too much drag. At 60 miles per hour you would get 29-31 mpg. Drag increases exponentially and is very noticable over 65 mph
. A hybrid has very limited benefits if mostly highway. The civic hybrid is an exception with its small engine and mild hybrid part.
At those high speeds I am not that surprised. I would hope at 65 miles per hour on a long trip the hybrid would get 28 mpg. My miles have all been a mix and 28-31 is the yield.