I'm sure there are stats on how much energy it takes to make a new vehicle, but there are several forces at work.

One is to reduce demand relatively quickly to bring down the price (assuming that's what is keeping oil high). It is hard to quantify how much making oil cheaper in Africa is "worth" under these circumstances.

Another is to give a boost to auto production which is (still) a big factor in the US economy when there is a downturn. This would also provide an incentive to automakers to switch to more efficient production sooner than the new standards mandate. This is also hard to quantify.

California had a program to buy back old junkers that were on the road a few years ago to help lower pollution. I think this was regarded as a "good thing" without a rigorous cost/benefit analysis.

Policies not Politics
---- Daily Landscape