WASHINGTON -- So now we know: The price point is $4.

At $3 a gallon, Americans just grin and bear it, suck it up, and, while complaining profusely, keep driving like crazy. At $4, it is a world transformed. Americans become rational creatures. Mass transit ridership is at a 50-year high. Driving is down 4 percent. (Any U.S. decline is something close to a miracle.) Hybrids and compacts are flying off the lots. SUV sales are in free fall.

The wholesale flight from gas guzzlers is stunning in its swiftness, but utterly predictable. Everything has a price point. Remember that "love affair" with SUVs? Love, it seems, has its price too.

America's sudden change in car-buying habits makes suitable mockery of that absurd debate Congress put on last December on fuel efficiency standards. At stake was precisely what miles-per-gallon average would every car company's fleet have to meet by precisely what date.

It was one out-of-a-hat number (35 mpg) compounded by another (by 2020). It involved, as always, dozens of regulations, loopholes and throws at a dartboard. And we already knew from past history what the fleet average number does. When oil is cheap and everybody wants a gas guzzler, fuel efficiency standards force manufacturers to make cars that nobody wants to buy. When gas prices go through the roof, this agent of inefficiency becomes an utter redundancy.

At $4 a gallon, the fleet composition is changing spontaneously and overnight, not over the 13 years mandated by Congress. (Even Stalin had the modesty to restrict himself to five-year plans.) Just Tuesday, GM announced that it would shutter four SUV and truck plants, add a third shift to its compact and midsize sedan plants in Ohio and Michigan, and green light for 2010 the Chevy Volt, an electric hybrid.

Some things, like renal physiology, are difficult. Some things, like Arab-Israeli peace, are impossible. And some things are preternaturally simple. You want more fuel-efficient cars? Don't regulate. Don't mandate. Don't scold. Don't appeal to the better angels of our nature. Do one thing: Hike the cost of gas until you find the price point.

Unfortunately, instead of hiking the price ourselves by means of a gasoline tax that could be instantly refunded to the American people in the form of lower payroll taxes, we let the Saudis, Venezuelans, Russians and Iranians do the taxing for us -- and pocket the money that the tax would have recycled back to the American worker.

This is insanity. For 25 years and with utter futility (starting with "The Oil-Bust Panic," The New Republic, February 1983), I have been advocating the cure: a U.S. energy tax as a way to curtail consumption and keep the money at home. In this space in May 2004 (and again in November 2005), I called for "the government -- through a tax -- to establish a new floor for gasoline," by fully taxing any drop in price below a certain benchmark. The point was to suppress demand and to keep the savings (from any subsequent world price drop) at home in the U.S. Treasury rather than going abroad. At the time, oil was $41 a barrel. It is now $123.

But instead of doing the obvious -- tax the damn thing -- we go through spasms of destructive alternatives, such as efficiency standards, ethanol mandates, and now a crazy carbon cap-and-trade system the Senate is debating this week. These are infinitely complex mandates for inefficiency and invitations to corruption. But they have a singular virtue: They hide the cost to the American consumer.

Want to wean us off oil? Be open and honest. The British are paying $8 a gallon for petrol. Goldman Sachs is predicting we will be paying $6 by next year. Why have the extra $2 (above the current $4) go abroad? Have it go to the U.S. Treasury as a gasoline tax and be recycled back into lower payroll taxes.

Announce a schedule of gas tax hikes of 50 cents every six months for the next two years. And put a tax floor under $4 gasoline, so that as high gas prices transform the U.S. auto fleet, change driving habits, and thus hugely reduce U.S. demand -- and bring down world crude oil prices -- the American consumer and the American economy reap all of the benefit.

Herewith concludes my annual exercise in futility. By the time I advocate the tax floor again next year, you'll be paying for gas in bullion.