I don't think that gas prices will have that much effect.
If you figure that most people buy a $30,000 car and use it no more than 5 years and sell it for say $10,000, they have $20,000 or $4,000 per year in depreciation alone. You add in insurance at $1000 per year (fast guess) and repairs for another $1000, you get $6,000 per year. If you figure 15,000 miles per year at 15 mpg, you have 1000 gallons and at $3 per gallon, gas makes up $3000 per year or only 33% of their vehicle's cost. Gas has to go to $6 per gallon to come to 50% of the vehicle's cost. Also, I see plenty of people driving large SUVs that get very poor mileage and are expensive to boot and are obviously never driven off road. I mean why does someone need an SUV to drive on clean pavement? I think people are hooked on large powerful vehicles as status symbols and it will take some doing for them to give them up. Something that might work would be for ultra high eff cars to become status symbols and you see that some with the prius.