Calculate CO2 produced
 

How about adding a feature that shows how much CO2 is produced from the mileage obtained. It is estimated that 1 gallon of gas produces 19.4 lbs of CO2.

So for instance, if my car gets 30 mpg and I fill up with 9.5 gallons..my car just produced (9.5*19.4=184.3 lbs of CO2). The car would have traveled 285 miles and thus you could get (184.3/285 = 0.65 lbs of CO2 per mile)