Bob Wilson posted a chart for a 2010 Prius but I think it is a base model and not a V. On that chart it shows a drop in mpg of approximately 12 mpg from 70 to 80 mph.
West Tx speed limit 80 mph our 07 got 38mpg 420 miles range, fully loaded. When we did 65 at night we got over 45mpg. If one added just a 15 gallon tank to their prius they could go from water to water Savannah Ga to San Diego Ca making just ONE stop for fuel if they started with a full tank and drove 65 mph. The wise thing to do would be to use fuelbuddydotcom to find the better prices absorb all you can in the best priced areas go into the RED zones and drive out without overpaying for fuel.
Driving between 75-80 mph with occasional slowing down to 65-70 ---> 35 mpg (800 miles) LA highway (Stop and Go - 60 mph) + Local traffic ---> 42 mpg (400 miles) 4 persons in car (450 pounds).
Last weekend coming south out of Dallas on I-45. About sixty miles of 65mph zone and we were getting average of 46.5 via computer. Most of the rest of the trip (about 180 miles) was at 75-77 mph and the computer showed 42-43 mpg. No wind to speak of. Dry roads. Temps around 60F. Two people in car with close to 200 pounds of cargo.
