Proposed "pay by the mile" tax a "disincentive to buy fuel-efficient cars"?

Okay, somebody explain this to me.

According to CBS (via Slashdot), California is thinking about eliminating the tax on gasoline in favor of a tax on how much driving you do. This is to make up for the fact that fuel-efficient cars mean people are buying less gas, meaning less taxes are received for road maintenance.

Privacy advocates say it's more like big brother riding on your bumper, not to mention a disincentive to buy fuel-efficient cars.

"It's not fair for people like me who have to commute, and we don't have any choice but take the freeways," says Just. "We shouldn't have to be taxed."

How is this a "disincentive"? You would still be paying for gas. A fuel efficient car would mean you'd have to buy gas less frequently. This means you would save money--the money you would have spent on gas. You just won't save as much, because the tax will add to that. But think of what people who have normal cars will be spending for gas...and they'll be taxed for distance too! How, I ask again, is this a "disincentive to buy fuel-efficient cars"? Would the savings on gas prices be that negligible? How much of California's gas price is tax?

What this tax would do, in my opinion, is encourage people not to drive. Of course, here in America, we don't always have an alternative.

Of greater interest to me is the technology behind the tax. GPS systems in every car that track your distance could eventually be used to, say, automatically give you a speeding ticket. The technology would definitely help if a car was stolen, or if you got lost. But at the same time, do we really want our locations available in real-time to anyone with access to the system?

Something to think about.