[dattaway]

Only problem is hydraulics aren't very efficient and waste much heat. They claimed a recovery of 1 kilowatt, which is just over one horsepower (746 watts by definition.)

Now if that vehicle has a 200 horsepower gas engine and is boosted by a 1 horsepower, a 10% claim might be stretching it. An older generation Toyota Prius has a 10 horsepower electric motor, which can propel it over 30 mph. One horsepower might be good for about 5 mph. I suppose there might be unusual driving conditions that might realize a 10% increase in mpg, but not in our world.

[Retric]

The average car needs ~25HP to maintain highway speeds. So 1hp could be ~4% of the car's energy needs. It takes a lot of energy and HP to accelerate quickly but you don't spend a lot of time accelerating like that. At lower speeds over bumpy roads I guess 10% might be possible.