This interesting story (the subject of which is not far from here in Montgomery) gives some general guidelines:
Though drivers across the nation are smarting from the rising price of gas, it is taking a particularly harsh toll here in Wilcox County, where the median household income is $17,500. A recent report by the Oil Price Information Service estimated that residents spend more than 13% of their monthly income on gas -- the highest ratio in the nation. (The study, which also took into account local gas prices and commuting statistics, found that the average Los Angeles County household spends 3.9% of its monthly income on gas.)
