At 75 mph the Prius gets about 41 mpg and at 80 mph it's down to 36-38.
The Volt, once the battery is depleted, gets about 34 mpg at 80 mph.
I see your point regarding the hybrid vs 4Runner mileage!
When I drove to Florida with 3 kids, a roof box, and two bikes on top of the Prius I saw 25...