For this to work, they'd have to retrofit
every car on the road with a GPS based device to track this. It wouldn't be fair to tax someone who drove across the country and back, based strictly on mileage. Can you imagine the bill at that fill-up?
What about people who live out of state, have an older car without the tracking device, who regularly do business in California? You can't track them, can't tax them, yet they may be using the roads more than the locals. What about private roads? It wouldn't be fair for them to tax some farmer who mainly uses a particular vehicle to tool around on the farm with. What about filling up gas cans? They don't have odometers, or computer interfaces. There are already some people who live near the state line, who cross over into a cheaper state to buy gas, as big as California is, though, it's just a drop in the bucket. But if Cali does this and the neighbors don't, people will be crossing INTO Cali, because there isn't a per gallon tax and they don't drive enough miles in CA to worry about the other tax. I'm sure the much smaller neighboring states will love that.
In all likelihood, this proposed tax, if passed, won't be used to
replace the existing per gallon tax, but to augment it, causing the double tax, as Alex pointed out. That being the case, what's the point in buying a hybrid afterall?