very happy with highway mpg

It would seem the reason you cannot simply average MPG numbers has to do with the fact that MPG is a ratio, and remember that you cannot add two fractions unless they have a common denominator. So, even though the miles (numerators) are the same, the gallons (denominators) are not, i.e. the fuel used varies over the same distance (round trip). When you have L/100km, distance is in the denominator, so over the same distance, you can compare, add, subtract or average the fuel usage just like regular numbers.

This comes up often for me, since I often get 60 mpg on the way to work (downhill), and 30 mpg on the way home (uphill). The average is then 40 not 45. It can be disappointing, because it seems as if one poor trip more than cancels out a good trip.

This is a good case in favor of a volume/distance measure of fuel economy instead of a distance/volume measure.

Miles/gallon implies that you have a set number of gallons available and you would like to see how far you can go on it. Gallons/mile (or 100 miles?) implies that you have some set distance to travel and would like to see how little fuel you need to burn to get there.

 

An excellent explanation.

Since a fuel tank usually has a fixed capacity (ignoring the Classic and 2G Prius fuel tank bladder for the moment) it seems that miles / gallon is an appropriate measure, at least in helping you to assess the vehicle's range before refueling is required.

 

OK, Now I'v got it. Thanks for the very clarifying answer. The neglect of including total gasoline consumed in round trip causes the error, correct?