Let me know what I am missing here, these are just some swags, but this is what I'm trying to get across, lots of places to lose energy, and 5hp isn't much to begin with, if accelerating or cruising.
Also keep in mind that plenty of folks here get 70mpg+ around town in metro sized vehicles with little more than technique. I get 60mph regularly in my saturn in town and can get 45mpg from here to california.
Perhaps it is because you are starting with 5hp, but it doesn't seem to add up to savings, certainly it does not add up to continuous highway operation or continuous repeated acceleration at every stoplight. The starter motor looks like 15shp. If I'm close with the losses, you will need to spend 10x as much time recharging with the 5hp wide open, as you do accelerating to be "continuous"?
BTW, why is this in fossil fuel free?