The irony is that engine and vehicle technologies have improved steadily in the past 20 years, and vehicles have become more efficient. But without either a push from CAFE standards or a pull from soaring fuel prices, the higher efficiencies are routinely offset by the increasing size, weight, speed, and performance of many vehicles. According to EPA studies, the fuel economy of the new 2007 light-duty vehicle fleet would have been more than 20 percent higher if it had weight and performance characteristics similar to 1987 vehicles. The unsettling result is that in the last 20 years the average fuel consumption in new vehicles has not changed.
Breaking this trend now through stricter CAFE standards will not be free. Tighter standards would require manufacturers to increase spending to reduce vehicle fuel consumption. The question is whether consumers are willing to pay more for that. Even at today's gasoline prices, there is little indication that consumers are demonstrating a sustained preference for fuel economy over vehicle performance.
IMHO, resolving this performance-size-fuel consumption trade-off is the most important element if we are to move forward in any meaningful fashion on reducing automobile fuel use.