There are a number of efficiency improvements that might be implemented through retrofits to existing assets. A highly efficient continuously-variable transmission will improve vehicle fuel economy by about 20%. Regenerative braking, hydro-pneumatic or electric, might yield another 20%. BMW apparently improved the fuel economy of some of their engines by about 15% at least partly by replacing throttles with variable valve lift and timing.

The problem is, there's little interest in incremental improvements. Maybe that will change when shortages become widespread, but individuals would have to spend their own money for efficiency improvements whose greatest benefits would accrue to everyone. Much as with buying a hybrid now, the added cost is borne by the individual, but the majority of the benefit of reduced demand is distributed to everyone, because the price elasiticity of fuel demand is so low. IOW, a small reduction in fuel demand leads to a large reduction in fuel price, which benefits everyone, but the individual who benefits most may be the one who lets others spend money on efficiency but pays less due to the reduced demand that results from the improved efficiency.