Italian term (edited): prova di regolarità

The winners of such an event is the driver who consumes the least fuel, not the one who maintains the steadiest speed, though of course a steady speed makes you more likely to win. They have to complete the course within a certain amount of time, which prevents them from driving at extremely slow speeds.

Two examples:

"What’s an economy run? It’s an event designed as a real-world test of fuel-frugal driving techniques and car fuel economy. It’s a competition held within the speed limits. The trophy goes to the driver with the highest miles-per-gallon (mpg) on the mapped course."

"Many years ago I helped a friend win a Corvette Club fuel economy run in his '72 350 M/T. With no prep other than 50 psi and full Hypermile driving, he averaged almost 60 MPG. It's a major part of the unlimited F.E. contests such as the Shell Eco-Marathon where the most recent winner averaged almost ten thousand MPG."