I've skimmed through a lot of the thread, but there seem to be a few notionally related questions here, and a lot of in depth technical response that's sort of answering something a tad different....
Right; first off, the Torque Converter 'lock up' speed isn't road speed dependant, its engine speed and load dependant.
On the RRC it locks at roughly 2,200rpm under normal load, which equates to the 55mph in top gear, on a four speed ZF, through the RRC's Borg X-fer. Lower reduction offered by the LT230 on the Dizzy would make that 52mph in that car.
The ZF four speed has a 'direct drive' 4th gear, giving no reduction, making the box a lot more efficient, as there's no power going through cogs or clutches, when locked out in top.
The 2,200rpm 'latch' is anoying in normal driving, and I find particularly so when towing and trying to hold a steady 55mph on a long run, where it's constantly 'shunting' between slip and lock out whenever you come to a gradient or such.
For more 'normal' driving, where that same hunting problem would manifest itself trying to stay inside a 50mph limit.....
Try selecting 3rd rather than D on the selector.
By holding the box back to 3rd, it engages a slightly different set of shift points.
Round town or variable speed limit roads, you'll find that the car behaves a lot more responsively, and drives a lot better, than leaving it in D.
And with the box held in 3rd, it wont try shifting or be stirring the Torque converter, trying to hold 4th with slip, it will be locked out, working slightly more efficiently in 3rd.
Suggestion about adding an Over Drive, with an auto wont drop the revs back in top as it would with a manual, and I dont think that would actually be helpful any way.
IF you were to add an OD, all that would do, when engaged is raise the final drive ratio.
So, when you were in that 52mph shift hunt situation, engaging the OD would result in the engine seeing a higher load, and the gearbox trying to find more reduction to provide the demanded motive force.
In effect, it would simply make the Torque Converter slip more (Box would see the OD like it would a steep hill), and lift the change point to something like 73mph (If its a 25% OD ratio), which would mean that the box wouldn't 'lock out' at legal speeds.
So you'd not 'see' any real benefit from knocking back the revs via an OD unit, as the auto-trans would simply hold the lower gears longer.
Given the efficiency advantage of the ZF's lock-out 4th, I think it would probably be a negative advantage, and take a lot of faffing with the shift pattern to make work comfortably.
Which brings me to the other suggestion about knocking the revs back for 'efficiency'...... that bit of 'wisdom' works well on a naturally aspirated diesel or petrol engine.
But, on a Turbo diesel, the engine is actually working at its most efficient at higher revs when the turbo is most able to deliver useful boost.
THE most efficient engine speed occurs where peak torque is made, which corresponds to the engine delivering the greatest cylinder pressure from the charge going into it......
On a TDi that is almost EXACTLY at 2,200rpm where the Torque converter locks out!
Making the engine 'labour' lower down particularly at part throttle is NOT actually saving you any fuel, as you are forcing the engine to work in a region of its operating range well outside optimum.
As said, peak efficiency occurs where you are getting the greatest combustion pressure, and is directly proportional to engine torque.
Look at the TDi torque curve and the peak is at around 2,200rpm, and tails off beyond that.
HOWEVER, between tick-over of 500rpm and the peak torque of 2,200 the torque curve ramps from almost nothing, incredibly steeply.
Ie; the 'efficiency' of the engine, is worst at tickover, gets better very quickly, the harder you rev it, and the drop off, going up the rev range is actually less than the drop off coming down it.
In short, Turbo Diesels, and particularly TDi's actually work better the harder you 'thrash' them!
So, as far as the original question is concerned, and the 'annoyance' of box hovering with the TC slipping at around 52mph in top, and the desire to get it to lock out and stop slushing, and to try and find a bit more ecconomy;
Answer, as suggested, to my tutored mind would seem to be simply, leave it in 3rd.
The TDi is not a refined engine, and might sound and feel better making it labour at lower revs, but for what you want to achieve, making the motor more comfortable to drive and finding a little added ecconomy, use 3rd, let the engine rev, and tolerate the clatter.
As for engine braking, long decents and heavy trailers.........
Auto's dont have the engine braking of a manual, BUT, they do have engine braking.
If you are suffering from the trailer 'pushing' the car on long decents, even holding the box in 1 or 2, then my first suggestion is to look at the trailer over ride brakes!
As the trailer catches up with the car, the brakes should be being applied, and the only extra force on the tow hitch should be the spring force from the over ride return spring.... a few pounds force, not 'tons'!
If you are used to driving a manual, and particularly have become used to a diesel manual, and relying on the imense amount of engine braking they can offer, then I can easily understand how the lack of engine braking with an auto may be disconcerting.
Recently was helping a freind fettle thier 90 for its MOT and sorting out some brake problems.... as in they didn't HAVE any, that they hadn't even noticed, for the simple reason they'd become so used to using the engine braking, they rarely used them above walking pace!
Following them about, red lights on the back hardly ever come on in normal driving.
With an auto you have to use the left hand piano pedal, but as comment I noted, that is actually NOT a bad thing, and it does relieve the drive line of a lot of strain.
They do still offer engine braking, just not in such bucket load quantities.
In normal driving, you can usually spot an auto without having to look at the badge on the back becouse of how often the brake lights are on, however, once you have got used to it, you dont HAVE to be brake dependant.
People are often dont realise my cars are all auto, becouse the red lamps dont shine as often as many fitted to vehicles with manual boxes.
And in a trip not long ago round the lanes of the Welsh mountains, with a convoy of Disco's & Defenders there were two of us with auto's, myself with a V8 classic and a freind with a 300TDi Auto, and our brake lamps were less lit than many of the manuals!
They DO work, but you do have to 'adjust' to the way they work.
