When I was browsing an online site looking for a new (used) car, I stumbled upon one converted to run on wood. Reading up on the subject, I found a website discussing the details. Apparently one issue is that the wood gassifier doesn't like throttle changes, especially after low power demand there's a big delay before it starts producing maximum gas again. One idea they had was to make a bypass valve and a fan, to suck out the gas and dump it to atmosphere when the engine didn't need it all. Not good for mileage, and at some point some environmentalists would probably get upset at carbon monoxide getting vented like that... Anyways, wouldn't hybrid technology be perfect for this, the engine can continue running at full power, keeping the wood gassifier running at optimum operating point, charging a battery bank.. So now we just need a hybrid with a 5 litre engine, convert it to wood gas, and we'll have the perfect vehicle for a post-oil post-apocalyptic society :-D