^^^^My thinking is this: For the best mileage, shift as low in the RPM range as possible, while still using a reasonable amount of throttle (don't floor it, then shift at two grand, then floor it again....). As long as you are cruising at a constant speed, then you should be in the highest gear that does not make the engine lug (for most cases, this will be fifth...unless you are going extremely slow).
That will maximize mileage, but you won't have a whole lot of acceleration. The light is typically for the best mix of mileage and performance, so shifting below it will give you a little better mileage.
Zetec....is that like Vtec or something???
Zetec Crew #103/ATX Faction #27 (retired)