I converted my dump truck to run on woodgas. Starting with my woody gasifier (woodygasifier.com) I welded up some skids and installed the gasifier, then went to work on assembling the fuel delivery system. Once finished I had a woodgas manifold that looked like something out of the cartoon "Snorks" and still had use of gasoline for hill climbing power. Then it was time to take the beast down the road. Between the truck and the full load of dirt, we had to be pushing at least 10,000 pounds, maybe more.
You can learn more about this and other woodgas powered projects at http://www.victorygasworks.com