[selbyk]

And distance. As the distance to get from A to B increases, mpg becomes less important of a metric than total fuel consumption.

(1 mile / 60 mpg)/(2 miles / 90 mpg) = 0.75

Traveling 1 mile @ 60 mpg consumes 75% of the fuel as going 2 miles @ 90 mpg does.

Edit: *Assuming the route planning grand-OP mentioned to optimize for mpg goes significantly out of the way.