Your idea is well known and is being used even now in many new prototype gas/electric cars. In fact this idea is central to the way all diesel electric train locomotives work. They have diesel engines that power electric generators that power electric motors in the wheels. Most of the trains in the world today work like this.
More information can be found in Popular Science magazine, which contains many articles on this subject. A quick websearch produces several articles on hybrid electric cars, e.g.: http://www.eet.com/news/97/980news/hybrid.html. There's also the Electric Vehicle Association of the Americas http://www.eaaev.org/ Their "EVQuestions" page has links to everywhere about electronic vehicles.
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