Edwin Pawlowski wrote:
That's a big drop with AC. Both my cars have 3.8 liter engines and the
difference is no more than 1 mpg. Neither gets 42 mpg and struggle to get
30 on all highway

I believe most cars get better mileage on the highway with the AC
turned on and the windows closed than with the AC off and the windows
open. The added aerodynamic drag introduced by opening the windows is
worse than turning the AC on.

Best mileage is with the AC off and the windows up :-)