Thanks for the feedback guys. For me the commuting often involves getting on my bicycle and riding about six miles in 25 minutes. I haven't used my car in more than a week. If I have to go to Brooklyn by car it's usually bad. Lots of stops and slow driving. I don't go there often. Normally I try to use the big highways in the car and avoid rush hour. On the bicycle I sometimes use the Greenway bike/pedestrian path part of the way but it's probably a slower route most of the time.
I took a look at <www.99mpg.com> and I like the E+wheel concept. If the van is going 45 MPH while getting 75 MPG, you can do division and find out that the idling engine uses 0.6 gal/hr of gasoline. If you then multiply by 12.5 hp-hr/gal (6.25 lb/gal / 0.5 lb/hp-hr) you find out that a small engine should make about 7.5 hp with that fuel flow rate. That is probably not enough to make a Caravan go 45 MPH. I think it would need at least 12 hp. Something over 40 MPG may be possible at 45 MPH.
I was thinking about something similar to an Ewheel but powered by a hydraulic motor. There could be a hydraulic accumulator that stores braking energy and energy from the extra engine. I found a couple of stories about this but they had their original transmissions removed. I'm hoping that a simpler conversion is possible. Here are the stories:
http://www.motherearthnews.com/Green...s-Mileage.aspx
http://www.motherearthnews.com/Green...-Gasoline.aspx
Here are a couple of stories about using a small engine and an electric motor:
http://www.motherearthnews.com/Green...ectic-Car.aspx
http://www.motherearthnews.com/Green...Hybrid-II.aspx
This last one has a diesel engine on a trailer. It can be disconnected for short trips.
Now here's a story about a pickup truck modified by the same guy. He says it can get 90 MPG.
http://www.motherearthnews.com/Renew...ctric-Car.aspx
Andy