a gas tax provides immediate, direct incentives for drivers to reduce gasoline use, while the efficiency standards must squeeze the reduction out of new vehicles only. The new standards also encourage more driving, not less.
Context: people keep telling me, whenever the subject comes up “but look how fuel efficiency has increased over the last 2-3 decades, it must be due to fuel efficiency standards” (see comments here for the latest repeats). I invariably reply: “but fuel prices have also increased a lot, how do you disentangle the effects?” And they repeat “Oh, yeah, maybe, but fuel efficiency standards are a really good idea because… obviously” (I may have slightly simplified the discussion, you understand).
This new study also provides something new to the discussion, which I’m sure a moments thought would have provided earlier (so clearly, no-one has been thinking :-): that new efficiency standards only help on new vehicles, which is a small proportion of the fleet, initially; whereas fuel taxes work for everyone.