There's a lot of talk about the fact that this would mean tracking everywhere you went. Why not just track miles driven by your odometer? I don't know about Oregon, but here you report your odometer reading when you pay for your license plate. You could just also pay the estimated tax (you already tell your insurance company how far you're probably going to drive) when you license your vehicle, and then pay/receive the difference when you re-register it next year, plus when you sell it. Seems a lot simpler and less expensive than a GPS tracker.
Also, seems like there should be a per-distance tax for roads, plus a gas consumption tax just to cover the increased societal costs of using gas. Use the gas tax to build out charging stations.