It makes sense to me because in highway driving your engine will operate continuously whereas in stop and go driving the truck will accelerate from a start using the stored energy from braking. I’m not so certain it will be as dramatic as claimed but these things have been in development for a long time and the system is currently running on vehicles such as garbage trucks and delivery trucks. Mileage improvement there is from 25% to 50%. The hydraulic hybrid is much more efficient than an electric hybrid at capturing brake energy.
It works in city traffic just fine and no doubt mileage performance will be improved. But the city mileage will NEVER be as good as the highway mileage. Some of that braking energy is going to go to heat and is not recoverable. The efficiency of the hydraulic recovery system can never be 100%. it could approach it but it can’t reach it.
I’m not knocking the system - just the claim that city mileage is better than highway mileage.