The Washington Monthly: Political Animal: Gasoline Alley
"It turns out that the average household uses about 1100 gallons of gasoline per year and has an average income of about $44,000. Take out 20% for taxes and that's a disposable income of $35,000.
So at two bucks a gallon that means the average household spends 6% of its disposable income on gasoline. At three bucks a gallon it's more like 10%. And that's only the average."