Cars are GEARED to get their best fuel economy at 55mph, because 55 is the average speed limit in this country, and in Europe.
You can gear that car, using cam specs, over drive gear sizes, and final drive ratios, to get maximum fuel economy for any speed up to about 85-90 mph, depending on aerodynamics. A car shaped like a brick is not gonna gain fuel economy at higher speeds no matter the gearing, due to the power output needed to push it through the atmosphere. Past 90 mph, even "slippery" cars need to be making decent power to maintain velocity.
Buy a BMW. It's peak fuel economy will be reached closer to that 80mph mark, because those crazy Germans like to drive fast.