I car that gets 30mpg on the highway only has an average requirement of about 50hp. If you had a large enough battery pack to smooth out the demand peaks and valleys there would be some benefit to having an engine running constant speed to provide that amount of power regardless of the fuel or even engine type. If the re-gen system could take advantage of a GPS system to anticipate impacts of upcoming terrain there would be even more benefits.
I believe that the Honda Clarity comes the closest to being that type of vehicle of any on the road right now. I was considering one but with only an 8 gallon fuel tank and a fairly small battery pack it just seemed to me that it would require too many stops. Plus the cost to benefit ratio just didn't seem to be there.
