This is kind of a random piece of data but I found it interesting.
I got the numbers below out of a report titled Household Income and Vehicle Fuel Economy in California That report is specific to California but its probably similar to national figures. Its quite likely that trends differ some based on region but probably not by a lot.
Here is the table :
|$0 to $25k||0.9||20.7||23.9|
|$25k to $35k||1||24.1||24.6|
|$35k to $50k||1.2||29||24.8|
|$50k to 100k||1.5||36.7||25.6|
People with higher incomes have more fuel efficient cars. I'm going to hazard a guess that this is in large part because people with higher incomes generally drive newer cars and newer cars have gradually better MPG ratings.