I have found the car often gets the same mileage at 60 miles per hour that it gets at 80 miles per hour but that really depends on various factors. Of course accelerating to 80 miles per hour costs more gas, so it does use more gas over short trips, but over long ones it seems fine.

The common tactic of allowing the car to slow down on uphills is of course extremely effective. Getting to a hill at 80 miles per hour, and allowing the car to slow to 70 miles per hour as it gets up it, saves alot of gas.

Certainly in terms of acceleration, the car gets far worse mileage at 10 miles per hour than it gets at 40 miles per hour, regardless of rate of acceleration. At high speeds, acceleration is typically at 20-30 mpg, but at low speeds it's between 2-10 mpg often.