According to this worksheet
Calculate HP For Speed
my 2010 Prius only needs 14 Horsepower to maintain 70 MPH on the flat. If I had a plug in rechargeable battery powerful enough to accelerate the car to cruising speed, (and help on the up hills), and let a small efficient diesel engine (say, a 16 HP Yanmar TS 190) [url=http://dieselparts.com.au/engines/ts190-yanmar/]
take over cruising duties, I wonder what Highway MPG I could expect.