Gotta love this guy!
This is the future: he's tweaked the Honda Insight hybrid so that you can manually control the gas engine/electric engine combo. Most hybrids are designed to conserve battery life, so they use the electric powertrain less. He says you can easily use it more or charge the battery more when going downhill, and get 50 mpg.
By adding a small extra wheel, you can have the electric powertrain run that only during highway cruising and get 99mpg!
It appears that Toyota and perhaps other manufacturers are going to do something like this, now that it is easy and battery life has been proven.
The gentleman is a contracting engineer doing this in his spare time. You can order kits to modify the Insight from here:
Here is his photo: