Originally Posted by
JDNSW
My experience is that the overdrive makes no appreciable difference to fuel consumption, possibly because it encourages you to drive faster than you would without it. The principal real use I have found for it is is conditions where you are driving in low range, but have an opportunity to go faster than is feasible in low fourth (but know you will need low again shortly), engaging overdrive enables you to stay in low range and not have to change into high range. Also useful for splitting gears, particularly for hills too steep for fourth, but not steep enough to really need third (but avoid use in first or second).
John