Years ago,my next door neighbor and I did a construction job in Jackson Hole,Wyoming.
For the trip home we rented a Chevy,similar to the Pontiac.My neighbor drove the whole way,sometimes at extra-legal speed (80 mph)and when we topped-off the car before returning it at the Dallas Airport,we'd averaged above 38-mpg for the one-way trip.
Seems like you'll see 1st-gen Prius HWY mpg with mods,no problem.
