there are two ways to look at this question, the best way would be to total all the miles on all the vehicles divided by total gallons they all used, divided by number of vehicles, or less accuretaly the average of all the vehicles added together and divide it by the number of vehicles.
our vehicles average around: 25mpg, 30mpg, 45-47mpg, and 45-50mpg, but the two 45mpg vehicles get most of the use, and that is not including motorcycles and my motorscooter.