How is 'Average MPG' Computed

[GoZoner]

Okay, the 'gallons consumed' include gallons for partial fill ups - which aren't used for MPG calculation.

But, another question:

It seems the 'Average MPG' for a car model year is not calculated correctly (See http://www.fuelly.com/car/honda/insight for the 2012 model year which shows 45.5 mpg). The average MPG should be calculated as 'total vehicle miles summed over all cars / total gallons consumed over all cars'. For the 2012 Insight this would be:

(/ (+ 2712.5 1217.4 1199.0 7415.0) (+ 57.95 31.13 25.78 159.71))

==> 45.69 mpg

The number shown, 45.5, for the 2012 Insight seems to be the average of the car averages. Computed as:

(/ (+ 46.8 39.1 46.5 49.5) 4)

==> 45.475

Here is an example of why 'the average of the car averages' is wrong. Consider two cars - one driver gets 50 mpg (say, 5000 miles using 100 gallons) and another driver of the same model car gets 30 mpg (say, 60 miles using 2 gallons). The 'average of the car averages' is 40 mpg. But, the average of the car MPGs is 49.6 [ (5000 + 60) / (100 + 2) ].