My comment was unclear; I didn't mean to say it was fuel rationing. I meant to say it was fuel-inefficiency rationing. (Unfortunately, "fuel consumption" is more easily interpreted in the way I didn't intend; I meant "fuel consumption per kilometer", a concept for which I will use "lpkm" below.) Each car is allowed to consume up to a certain lpkm, averaged over the manufacturer's offerings. It's true that a strict rationing scheme would simply stop sales rather than merely levying a fine, and that being stranded on the way to work is a worse outcome than either one.
There are two key aspects of the CAFE that are more ration-like than Pigovian-tax-like:
1. The fine is zero for manufacturers whose "fleets" are more efficient than the standard requires, so it provides no incentive to reduce lpkm below the quota.
2. The quota is averaged across all the manufacturer's vehicles, so it is in a sense a per-vehicle lpkm quota that the manufacturer can choose to redirect from some vehicles to others, so a luxury car maker can evade the "tax" by merging with an economy subcompact car maker.
There's a third aspect, though, that cuts in the opposite direction. CAFE is calculated on reciprocal lpkm, so I think adding excess lpkm to an already-inefficient "fleet" costs less than adding the same excess lpkm to one that's just violating the quota. So a car maker that greatly exceeds the quota can reduce the "tax" by splitting a division that roughly meets the quota out into a separate company.
I might be miscalculating that result though.