Droid13 — One's trip average FE (in L/100 km in the metric TCH) will be lowest if the distance-averaged (not the time-averaged) value of the instantaneous FE gauge reading is the lowest possible. This means, for example, that an instantaneous FE reading of 10 L/100 km for 1 km of distance travelled contributes to the overall trip FE the same amount as an instantaneous FE reading of 5 L/100 km for 2 km of distance travelled. It's not easy to optimize this in practice, however, but this is what one would have to do in order to use the least fuel for a given trip. There's no equivalent statement that can be made regarding how to optimize FE using the gauge (in mpg) in the US TCH, due to its reciprocal relation to fuel consumption.

Stan