You still have the weight of the fuel to worry about, though. A kilo of gas contains about 47 megajoules of energy, a kilo of wood only about 16.
That means that if you had a 20-gallon gas tank, and assuming that you had a steam engine that can convert wood to energy at the same rate an internal combustion engine (Which is a looooooong stretch), you would need about 160 kilograms of wood. That's about what a small tree weighs.
Still, it's something to consider if there isn't any petrol left.