It takes too much 'energy' - and that's a big word in this case - for a dinky one vehicle gasifier.
As Pat said, chop a tree down first and after you chop it into little pieces dry it out for a few weeks or months (chain saw gas, your personal bowl of Cheerios just to begin with). When it's all nice and dry, put it into the top chamber of a furnace (which of course you had to build, using electricity, MIG gas, etc) then light the fire below it so it gets nice and hot, driving the combustible out. Now, to be 'really efficient'(), you need to produce some quantity of that gas and store it under some pressure - so that's a compressor and a tank. OK, next, we not only have burned some fuel, we have made a little fuel - now we have some waste - that's the burned fuel ash and the top chamber leftovers, which, if efficient, is dust. Back to you and your morning Cheerios along with a shovel then off to where you can dump this leftover 'stuff'.

If you are lucky enough to own a saw mill, millwork shop or even a wood lot and supply firewood, then you might be able to make it (kinda) worthwhile.

If you really want to see the tech on this and feel the need, do a search with this as the set of terms: 'ge syngas technology' where GE actually did gas turbine power plants, plus have others that I have heard are being built and, in this case, proved that technology for large scale use http://www.ge-energy.com/content/mul...Technology.pdf