Googled hypermiling...found myself here ('17 Jetta 1.4T auto)

[oil pan 4]

The diesel engine wants some boost to be most efficient.
That way you have plenty of air to burn a little bit of fuel.
When I added the turbo on by diesel suburban I picked up at least 10% more fuel efficiency with out changing anything else.

If a gasoline engine is running boost then the engine is producing more power than it would at wide open throttle with no turbo.

[Gasoline Fumes]

Driving a rented 1.2L TSI Golf in Finland a few years ago, I noticed the instantaneous MPG on the factory instrumentation was the same no matter how I accelerated. So I used wide-open throttle to spend less time accelerating.

[oldtamiyaphile]

Originally Posted by Gasoline Fumes

Driving a rented 1.2L TSI Golf in Finland a few years ago, I noticed the instantaneous MPG on the factory instrumentation was the same no matter how I accelerated. So I used wide-open throttle to spend less time accelerating.

On some cars the display maxes out. Ie my Fiat maxes out at 25l/100km, even though in the lower gears at least it's quite capable of using more.

[MetroMPG]

Originally Posted by BSEM

Do you know where I can look to find info for the most efficient engine RPM when cruising in 6th gear of my particular car?

Not for your specific car, but almost every car I've ever driven (gasoline) gets its best fuel economy at the lowest engine RPM you can reasonably drive in top gear. That's a good rule of thumb to follow.

For many examples, see:

Thread: Speed vs. MPG charts (post 'em if you got 'em)

Also, I second the recommendation of picking up some additional MPG instrumentation. You ideally want to see both instantaneous and trip average.

Another thing to pursue: if your automatic transmission has a "manual" mode, you can probably request earlier upshifts than the default "D" mode, and get into higher (fuel-saving) gears sooner.

EG., related topic:

Thread: "Short-shifting" a Jeep Patriot & Audi A4 CVT (forcing early "upshift") saves gas

And... belated welcome to the forum!