What if?

What if we tried funding the U.S. miliary largely through as gasoline tax, how much would it cost per gallon?

Pretend military expenses ~$500B/year
Pretend 150 million cars, driving 12,000 miles/year, and 25 mpg.
That's 72 billion gallons.

So taxing gasoline at $6.94/gallon would approximately cover our military expenses.

Is there something wrong with this picture?