My new Honda Hybrid has a constantly variable transmission, which I imaging is a purely hydraulic system. If I shift into neutral going down significant hills I can increase my mileage by about 5%, or from 40 to 42 mpg (not quite as good as my old Harley). Am I doing any harm to the transmission by doing this? I have a 110,000 mile warranty, but it will take that long to break even for dollars I paid for up front (about 100 tanks of gas).
Think if the CVT as having one large gear that is shaped like a cone. The mating gear slides from one end of the cone to the other, making the variations inbetween much more subtle than a normal transmission shift. This is a pretty big breakthrough in efficiency. Now consider that if something happens to your CVT, it can’t be repaired. It can only be replaced. If this was a good idea, Honda would make the car do it for you. You just spent a lot of money on a very efficient car, and now you think you are qualified to come up with your own plan to increase the efficiency even more?
I am less concerned with the transmission than I am the other drivetrain components, including the electric motor. Those could also be hurt by doing this. Furthermore, your car is sophisticated enough that Honda will probably be able to tell you have been abusing the car. That will probably void your warranty, even if you keep paying for it.