Tonneau Cover+MPG

Seriously, the only three ways to truly improve MPG is:
1. More efficient engine and drivetrain
2. Reduce drag and rolling resistance
3. Go on a weight loss plan

You can solve #1 with an electric drive. But... range.
You can solve #2 by making the truck look like a Ridgeline on steroids. Lower it, make it look like a Prius from the front.
You can solve #3 by reducing payload and towing capacity.

Basically turn it into something that gets 35 mpg that nobody will buy.
The frontal area is the key to drag. Fix it and increase mpg. Adding or redesigning anything else will only give you tenths of a mpg.
#physics

 

Maybe OP is driving slower with the cover on so that it doesn’t fly off.