What is wrong with highway choice for cars. Highways are the most eco roads there are. The speed you choose makes the difference.
Highways are very economic only in sense you avoid acceleration that spends a lot of fuel. But high speed is very non-economic. Regarding chosen speed, Eco profile does not count with speed for motorway 130 km/h.
Most economic is steady speed somewhere is interval 70-90 km/h - it is car dependent. But nobody would run at this speed on motorway ( unless perhaps saving Dutchmen, in early 90s they had motorway maxspeed 80 km/h )
The specific fuel consumption at 120-130 km/h is up to 1/3 higher then at 70-90 km/h.
For Fast profile is expected you run on motorways as fast as you can, i.e. e.g. 130 km/h . ( US guys would have to tweak the numbers for now ).
For Eco profile is expected you run slower at 110 km/h to save the fuel.
See also Brouter Car profiles Github wiki