The US budget is something like 4 trillion dollars. But let's say we run a defecit or get money from other sources, and only need 2 trillion from this gas tax. There are about 250 million cars in the US. If they each average 12,000 miles/year at 30 miles/gallon, that's 400 gallons/year on average. To meet our 2 trillion dollar goal, we'd need a tax of (2 trillion)/(250 million)/(400 gallons) = $20/gallon. That doesn't sound too unreasonable until you see that it would be a 1000% tax on the price (about $2/gallon currently) and would make a 12 gallon fill up go from $24 to $264. No one, anywhere, would agree to that, even if it works out about the same, because the income tax is a hidden tax -- the money is taken before you get it (part of the brilliance of the system).
Besides, sales taxes like this are inherently regressive, and hurt the poor much more than the rich.