The neighbor to the north (WA) has a sales tax of almost 10%, but they are one of a handful of states that do NOT have a state income tax. That's why WA and OR are roughly comparable. One has sales tax, one has income tax. If OR added a sales tax, the financial burden on its citizens would be as oppressive as neighboring socialist hellhole to the south, CA. OR simply does not have the population density nor the amount of industry that would enable CA-levels of taxpayer-rape, and they certainly don't have the infrastructure to justify it. (most of OR is uninhabited desert and mountain ranges. Nearly everyone lives along the coast, the Columbia river, or the I-5 corridor)
The proposed vehicle usage increase is probably not about a lack of existing revenue to fix roads anyway. I'm guessing that they just want more money to spend on the kind of hippie, nanny-state bullshit they are famous for.