I doubt it yields better FE, unless the engine is overtaxed by being in a higher gear...however, given that it is an automatic I think "overdrive" would yield the best FE. Just concentrating on keeping the vacuum the highest in top gear and trying not to let it shift down. 35mph is tough in most automatics I've driven, though accelerating up to ~40 then easing off and letting it shift into overdrive usually allows it to stay in overdrive at 35mph unless you hit a hill. I have a '94 manual trans car I'm going to throw a vacuum gauge in soon to test it out. My newer car is an auto with a SGII and leaving it in "overdrive" (or neutral) always yields the best mpg. I'd be curious if someone here could compare SGII readings to vacuum readings. It's too bad the SGII doesn't have an O2 and vacuum readings.
Many applications of the Scangauge have Manifold pressure. It depends on whether the car has the sensor. Manifold pressure us sort of the inverse of manifold vacuum but essentially the same information. You can also infer a lot from the LOD gauge.
It's possible that the degree of slip in the torque convertor at lower revs is enough that you use more gas thrashing fluid than moving. Also some motors, at lower revs you can end up giving them more gas to keep them moving under load, than it takes to run them at higher rpm... like when you slog up an incline and keeping the same throttle position, race away at the top, not just a few rpm faster, but double.
I sometimes end up in the wrong gear in my van, if I coast down slow, perhaps to about 30kph without touching the brakes, I stay in 3rd, which when you try to move, unless you give it most of the pedal to kick it down, it takes a lot of gas to get it moving again. So I usually change down manually. I can avoid this by remembering to touch the brakes at least briefly while slowing. If the traffic is just moving off very slow, 30kph up to 35 kph I'll just try to coax it along carefully with a light touch on the pedal until it picks up again, but when everything takes off, I drop to 2nd to get it back up to 50, then shift it back to D.
The S-15 Jimmy we had a few years back used to do a city mileage of about 21mpg if you drove it in D and only 19ish if you drove it in OD.
My Escort though, the convertor must be more efficient, since I would get great city mileage when I got it into 4th (not sure if I got TC lockup) but it involved "cheating" somewhat, because I'd have to speed, go up to 60kph and ease it back down to 50kph to get it to stay in, but it would just loaf along at little more than idle. But that's not much use in stop and go, just on longer city limit stretches. It's good on those country highways where you'll get the road passing through a small town or village and the limit dropping to 50kph, you could ease it down without dropping gear and just loaf through.
Anyway, it's something that one would have to test with one's own vehicle, it's possible that with a vehicle with a lossy convertor and a motor that doesn't like low speed lugging will do better in lower gear, whereas motors with more low end and more efficient convertors or lower speed lockup may do a lot better in D or OD.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice