Fuel economy and cruising

I apologize for not sourcing, but I read a page that showed the most efficient speed for the 4.0 (or possibly most car engines) is between 2K and 3K.

I try to get the most from my tank and have been trying to cruise around 45 to 50mph, because of the overdrive and having RPM's about or a little over 1500.

If I downshit to 3rd gear while in drive I can let up on the throttle to maintain the same cruising speed. Would this indicate higher efficiency? Less throttle, less air, less fuel?

Given a situation, cruising at 50mph, 4th hitting ~1600 and 3rd hitting ~2100 would this situation be right;

4th gear, more throttle, more engine strain, less rpms = more fuel needed to maintain speed

3rd gear, less throttle, less engine strain, more rpms = less fuel needed to maintain speed

After all this mess pretty much all I'm asking is if you aren't going to go over 60mph, would it be more efficient to leave it in 3rd instead of drive? I guess the only problem would be descents where engine braking use more fuel.

I currently have a set of P235

And if this is true is there any way to defeat the D-3rd shift lock

Thanks

Engine braking doesn't use fuel, it just revs....power is being transfered from your wheels to the engine, not the other way around that would require the use of fuel.