With fuel economy its best to play around and see what gives you the best MPG. For instance, if im cruising on a 30mph road, 5th is better MPG then in 6th. And just remember, the heavier the car, the more fuel is used so make sure the boot isnt crammed with needless bits.
at 30 mph, you shouldn’t be anywhere near 6th (or top gear) , which is a super-tall overdrive ratio for highway cruising while the engine just putts along. to get better mileage, keep the engine revs at the beginning of the power band, or in newer cars upshift before the engine switches to high-power mode (vtec, etc.)
the effectiveness of engine braking diminishes with lower revs, so keep the revs higher (lower gear) than if you were accelerating. *not responsible for burnt clutches or other breakage*
in many u.s. states, canadian provinces and so forth it is against the law to keep a car in neutral while operating.