http://evalbum.com/build#mowerpower
"...most attempts to extend the range of an elecctric vehicle this way creates something that actually causes more air pollution than a conventional car and
gets poorer fuel economy....
A number of folks have put together range extending trailers using small car engines to drive large generators. In most cases these seem to work quite well, but the fuel economy
is not dramatically better than a conventional gas car..."
If budget is a concern, hypermiling is your best friend.