From the Department of Wretched Excess comes a television advertisement that exhorts men to "restore your manhood" by driving a giant Hummer sport utility vehicle.

Playing as it does on testosterone intoxication, not to mention the turgid sense of identity and entitlement many Americans attach to their daily mode of transportation, the ad reflects the reality that gas-guzzling cars remain very attractive despite $3-a-gallon fuel.

That's why a proposal in the state of Oregon to tax motorists on the basis of mileage rather than the amount of gasoline they consume is a bad idea. It would kill any incentive for motorists to choose fuel-efficient vehicles in the interest of weaning this nation from its shameful and dangerous dependence on imported oil.

Like many states, Oregon is trying to find a solution to static revenue from its gas tax, which cannot keep up with the ever-increasing cost of highway maintenance and construction.

One idea, now being tested in a pilot project involving 280 volunteers, is to substitute a highway "user fee" of 1.2 cents a mile for the state's gasoline tax of 24 cents a gallon. Mileage driven inside the state would be logged via global positioning devices installed in volunteers' vehicles.

The threat of Big Brotherism raised by the potential for continuous satellite tracking of motorists via GPS has resulted in objections from wary Oregonians, but little attention seems to have been paid to a greater danger -- the likelihood that such a system would destroy the conservation ethic.

What better way to destroy that ethic than to impose an equal road fee on drivers of, say, a Toyota Prius hybrid that actually gets 50 miles per gallon around town and a Hummer H2 that gets, at best, 10?

Yes, we know that people like Vice President Dick Cheney regard conservation in the form of good gas mileage as merely a quaint personal virtue. But it will turn into a national mandate come the day that conflict in the Middle East or some other disruption shifts the oil crunch from price increases to supply limitations, as in the 1970s.

Waiting in gas lines, many motorists undoubtedly will wish they were driving a vehicle that emphasized miles-per-gallon instead of off-road capability.

A per-mile tax also would be inequitable from the standpoint that larger, heavier cars inflict more damage on roadways than lighter ones. That principle is reflected in so-called axle-mile taxes paid by truckers, in addition to fuel taxes, for highway upkeep in various states. Why shouldn't the same apply to cars?

All motor vehicles are not created equal, and any solution to better-maintained highways like Oregon's proposed mileage tax should not treat motorists as if they are. Drivers who consume the most gasoline and batter the pavement into gravel with heavy vehicles should continue to pay a proportionately higher share of the bill.