New cars might be getting more efficient, but if there's an economic slowdown (and the Fed now seems to be getting trapped into more interest rate rises by the inflationary pressures that are due in large measure to energy prices), reduction in car sales will limit the benefits.

IMO this is a very important concern, and it goes beyond vehicle mileage to energy efficiency generally. The world has been awash in liquidity lately, but that state of affairs appears to be unlikely to continue. It isn't just the Fed that one has to watch (see for instance The Nightmare Carry Trade Scenario). I would argue that credit tightening is both inevitable and imminent, and that it is likely to have a very significant effect on the availability of resources for funding efficiency improvements on all fronts.

US consumers are over-stretched and any further tightening could easily push them over the edge, particularly in relation to housing. Altering structural inefficiency is an expensive business. People who are underwater on their mortgage and struggling with credit card debt are unlikely to be buying more efficient vehicles, even if the fuel consumption of their current vehicle is crippling them. The same argument would apply to replacing inefficient appliances or installing improvement such as solar domestic hot water. Rather than increasing efficiency, people are likely to be forced to keep using their current vehicles and other energy-consuming items, but to use them less as energy prices rise relative to other costs.