Regulation is always tricky. The U.S. CAFE standards was raised on cars, but not trucks partly helped to inspire the SUV sales boom. And which is worse a minivan getting 20mpg with 6 people in it, or a hybrid with a single person in it getting 45mpg?

I don't know what will make the biggest difference, but can't imagine any regulation can beat weathy people's desire to consume - it might as well redirect their wealth to something even worse for the environment.

I've long been on the high "sin tax" approach, even as I acknowledge it is regressive, hurting those least able to change, and having little on effect on those well enough to pay "any price".

The only other alternative I know is rationing. Combining rationing and ebay could make for an interesting market! The poor can have their share if they need it, or make a few bucks for finding a way to live without a car, or driving less.

I don't know much about rationing, but would imagine coupons would have expiration dates to prevent hoarding, and limit "inflation" of value, but expirations could still excourage fuel hoarding itself, ESPECIALLY if the ration levels are decreasing. A fun game, however played.

More effective would be a tiered excise ... at a set rate for vehicles with higher fuel inefficiency (l/km, g/m) than the current fleet average, twice the set rate for vehicles with twice the fuel inefficiency or more, no excise for vehicles from the fleet average to half the fleet average inefficiency, and a subsidy at the set rate for vehicles at half the fuel inefficiency or less.

And surplus on collections over the cost of the subsidy going to buy more energy efficient running stock for public transport.