Oregon has announced it is implementing a plan to tax motorists based on miles driven (Vehicle Mileage Tax, or VMT), not on fuel consumed. Whether a good or bad idea, here are some things to consider:
Currently, the amount of fuel tax that someone pays depends on (1) the car's weight, (2) fuel efficiency, (3) time in traffic, and (4) total length driven. In other words, if you drive 3000 miles in a Hummer, you will pay more tax than if you drive 3000 miles in a hybrid. Similarly, if you drive 3000 miles in traffic, your will run out quicker than driving 3000 miles at peak capacity, and need to fill up more and pay more fuel. That is an incentive to get a more fuel-efficient car, and avoid traffic.
Under the VMT, a hybrid driver will pay the same per mile as a Hummer driver. If the VMT is revenue-neutral, Hummer drivers would pay less than they do now while hybrid drivers would pay more. However, it likely will be revenue-positive, as part of the reason is to make up for the reduced fuel-tax revenue due to greater fuel-efficiency. Therefore, it's likely everyone will pay more, but hybrid drivers would likely see a greater % increase in their bills than Hummer drivers. This removes an incentive to get a fuel-efficient car, and avoid traffic.
Proponents of the VMT do not dispute any of that, but they argue that the wear and tear on the road is similar, whether you drive a Hummer or hybrid, and that hybrid drivers are not paying enough to make up for that wear-and-tear. That is likely true, but you will need to factor in whether that outweighs the current incentives to get a more fuel-efficient car, and to avoid traffic.
Furthermore, a legitimate question exists regarding how to track miles driven in-state. If government is to just go by your odometer, you will also be taxed for miles you drive out-of-state. One idea is to require a GPS in cars, so government can accurately subtract miles driven out-of-state; that raises legitimate privacy concerns. Some argue that your phone gives people more information than the GPS would, but a fundamental difference exists: if you do not want anyone to know where you are, you can turn off your phone or leave it somewhere. By contrast, you won't be allowed to turn the GPS off.
Additionally, drivers from out-of-state will get a free ride on in-state roads; Oregon cannot tax drivers of cars registered in other states, but it can tax anyone for buying fuel in-state. Giving out-of-state drivers a free ride cannot be justified even from the perspective of wear-and-tear on the road, unlike the hybrid and Hummer issue.
In short, the ones who will likely pay more under a VMT are in-state fuel-efficient drivers, and those who will pay less are out-of-state drivers, regardless of their type of car, miles driven, or anything.
Finally, the current plan is "voluntary," meaning you could choose either to have your vehicle miles taxed or your fuel use taxed. People people act rationally and will choose the one that costs them less. The problem is those who drive heavy gas-guzzlers and/or in traffic a lot will choose the VMT, while others will stick with the fuel tax. That will result in even less revenue than now, thereby defeating the purpose of having people compensate for wear-and-tear on the roads.