memmel,
Thats an interesting graph because in the 1978-85 period when car sales($ value or number?) declined is when vehicle fleet mpg increased the fastest in the last 50 years. If you think about it, most fuel efficient cars( except HEV's) are much less expensive. I am not sure about prices in US or Europe, but in Australia, smaller cars using 6L/100km cost about $AUD 15,000, while SUV's and light trucks getting 9-14L/100km, are about $AUD 35,000 or more. In other words a family could rather than buy a new SUV, replace both fuel inefficient cars with two new cars, use about half as much fuel for each car and still spend less. If its not happening yet in US, wait until prices are >$8 per gallon. Just about every new-car buyer can do the sums( small car=saving >250 gallons of fuel per year, plus half the financing costs).
Its more complex for second-hand car buyers, but if you look in papers lots of older fuel efficient cars available <$5,000.
The issue about paying off existing cars is important, but many new cars are on 3 year lease arrangements. Its not as though consumers are spending 100% of disposable income on vehicles. In The Australia Financial Review it showed a graph that shows that vehicle running costs as % income are only 6%( was 8% in 1980). Even a further doubling in fuel( to $3.50/L;$USD 12/gallon) could be accommodated by reductions in eating out, reductions in electrical, furniture and white goods purchases.