[slowbro]

I think it makes more sense when you look at speed.
If I drove 30 mph for the first half of a 30 mile trip, and 15 mph for the second half, my average speed is NOT (30+15)/2= 22.5 mph.
Instead, it took half an hour for the first half and an hour for the second half, so you're average speed is actually:
(30 miles)/(1.5 hours)= 20 mph.

The key is in the denominator you cannot add 30 mph to 15 mph, because even though the miles driven is consistent, the time taken is not. You have to find the least common denominator to add them together.

However,
If I drove 30 mph for 2 hours and 15 mph for another 2 hours.
(30 mph+15 mph)/2= 22.5 mph does work because the denominators (time) are the same.

You can check that:
(60 miles driven + 30 miles driven)/4 total hours= 22.5 mph, which is the same.

So yes it would make sense to switch to (fuel used/mile) since our commute distances generally stay the same. A one mile road tends to not change in size from day to day. What a strange world that would be.