I've mentioned this before on TH. The Earth is already stretched for agricultural space. Using agricultural waste is ok for biofuels and bioplastics, but chopping down forest to fuel our cars gets us nowhere. We need to be careful of how much we push such things, or we will create as much harm as we hoped to eliminate.

Demand for fuel, plastics and food will grow despite how much we 'Huggers' un-pimp our rides, because the world population is expanding so much faster these days. Convincing governments and entire societies to take action to reduce population growth will be far more difficult than choosing a different car.
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Carl's assertions are correct as far as they go. The original post dealt, however, not with logging existing forests per se but with willow or kenaf plantings, for eexample, for the specific purpose of creating feedstock for biofuel production. This could be done on marginal lands that are either droughty or erosive when ploughed. Hope this clarifies.

Your comments on cutting power output are probably a bit too simplistic and are unlikely to be widely supported by the public as a whole. Furthermore, one could argue that those people unconcerned by a reduction in power loss are already going to be driving the lower powered versions of the manufacturers cars.

Apart from power output automotive weight makes a considerable impact on fuel consumption, and average weight has certainly increased since 1990. If we could reduce the weight of a car by 50% at the same time as reducing power by 50% consumers would see little change in the cars performance. Lighter cars are often more responsive and 'fun' to drive as anyone who driven Lotus' 720kg Elise will testify (120kg lighter than a Smart car)

There is an excellent 6 page survey in today's Financial Times mostly about environmental concerns and the motor industry that is worth reading. It discusses most fuel types and the issues with adoption.
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Excellent comments. Notably, when overall vehicle weight is reduced, far less horsepower is needed to deliver the same torque. I take the long view on engine power. Inside of 50 years people who enjoyed fast cars will have to see them in museums or join private racing clubs (already fairly common). It only took a decade for the fad of big engines to take off; and I see the tipping point on reversing engine power led by the introduction of powerful and light hybrid configurations that achieve high initial torque with less overfall HP and double or more the fuel efficiency.

Perhaps it should also be mentioned that modest reductions in meat consumption could free up a lot of the land used to grow grain to feed the meat. If biofuels become cost-competitive with fossil fuels on their own, this might even happen automatically, if growing fuel with a given spot of becomes more lucrative than feeding meat with that same land. I suppose overall this would probably be environmentally neutral in terms of the effects on the agricultural lands themselves, but it could mean slightly reduced net carbon emissions, and health benefits to the population if excessive meat consumption became too expensive and reduced-meat and vegetarian/vegan diets become more prevalent.

Another point that needs to be made in the whole biofuel debate is that countries such as the Uk do not enjoy comparative advantage in feedstock production. Developing countries which do enjoy comparative advantage should be encouraged to look to biofuel production as a way of bolstering failing agricultural industries - I am think here of the sugar sector which is unfortunately suffering in many ACP countries due to the withdrawal of the EU stabex support scheme. Biofuel production could provide a welcome opportunity for diversification