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The idea of attaching a motor to the unused differential of a 4x4 has come up a number of times, so a search could probably answer more questions than I can.
The first thing to consider would be losing the SUV. You'll save the most money by selling the SUV and buying a cheaper, more economical car in cash. Otherwise, you'll have paid for an expensive car and an expensive modification just to reach parity with something that can be had cheap to begin with...
Secondly, stop lights can be pretty well dealt with by maintaining momentum. People have been known to pass through over a 100 stop lights on a trip without using the brakes once. It will take effort, but it is free. Technology has shown itself to be a handicap for incompetence (look at the Prii speeding on the freeway).
I'm not the most knowledgeable on electric cars, but I believe DC motors do not technically need controllers. A potentiometer could do. Personally, I would try to calculate the possible savings first through detailed testing and theory.
The constantly spinning motor could be solved through a sprag clutch. It would probably take some unique engineering, though.
Overall, it's feasible but impractical. Why set your sights on Everest if you're a novice mountaineer? Take the cheap, low hanging fruit first. You'll be surprised how much you can save by changing your driving habits and performing a few basic modifications.
- LostCause
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