The other day I saw the start of a home built plug in hybrid truck and it looked like it was well on it's way to being put on the road, the only thing that I did not like was that it was going to have two throttles!
So my question is focused on the OBDII port of a Ford Ranger pickup truck, because that is the vehicle that was being converted and my parents have the same vehicle and if I can make something that works I think they would let me make their truck a plug in hybrid, or at least my mom asked when I was going to start that kind of project with their truck
My two lines of thought are to either go with a simple and crude speed controller, basically a vacuum diaphragm that would control a mechanical throttle pot box for the electric motor, this would be the same idea and driving with a vacuum gauge only the electric motor would be kicking in under low vacuum.
The other idea is to use the OBDII port and have something like an Arduino monitor engine load, although I suspect that is more or less looking at vacuum as well? but with that it would be all electronic so tweaking could be done with programming instead of with springs and hoses and if done well should, in theory be more reliable.