There is always the famous TOPGEAR test
BMW M3 Beats Prius in Fuel Economy Test
Where a BMW M3 had better fuel economy than a Prius on a closed track. However the Prius was maxed out on speed and the BMW was barely taxed.
I think alot of economy at high speeds depends on gearing and where the engine/drivetrain was designed to operate.
A 1974 law (in the US) instituted a national speed limit of 55 miles per hour (a compromise between efficiency and speed). But as the oil crisis abated, the law was amended to 65 miles per hour in 1987 and finally repealed entirely in 1995, ceding the power to set speed limits back to the states. Now, many states have speed limits that exceed 70 miles per hour on interstates, and some stretches in Texas and Utah have limits as high as 80.
So older cars from the 70's and 80's may be designed for 55 MPH best fuel economy.