That would probably annoy 4x4 drivers more than making them think about their consumption. A better way may be to require expensive commercial registrations for all light trucks and maybe offset it with a tax credit for documented business use (cargo or multi-passenger use only). Even that would be pretty unwieldy in practice, but it would make people think about their choice of driving an off-road truck as a private passenger vehicle. Boaters and off-road enthusiasts would be screwed, but that's the price of a "lifestyle choice".

After I typed the previous comment I forgot to clarify I meant 4 inch by 4 inch sticker. I belatedly realised that many outside the US call SUVs 4x4s.

That's funny. They were called 4x4s in the US too, but the marketers needed a new name that sounded less truckish and more appealing to soccer moms. I remember when they first started using sport utility vehicle in the early to mid 90's and it sounded very awkward.