In response to this item on mileage-tax musings, Planet Gore reader Dr. Bob writes in with the line of the day [emphasis mine]:

When people drive more fuel efficient cars, government realizes a projected tax-revenue shortfall. We can’t have that, or course!
Now that it will cost the same per-mile to drive a Hummer/Escalade/Winnebago as it does to drive a hybrid, why would anyone ever consider driving a penalty box?
Of course, this is supposed to replace the gas tax; we all know that it will actually supplement the gax tax. I await more government taxes with my Astroglide at the ready.