Towing in top gear

You should keeping out of top gear when going downhill with heavy loads so the trailer doesn't push to vehicle too fast as breaking heavily (especially when going downhill) can jack-knife you rig.

5th gear is an overdrive gear and not as strong as 1 - 4, so as in most cases the box trailer to the tip is ok in 5th but your van/2 ton camper 4th is the go so as not to put undue wear on the 5th gear IMHO.

Most mechanics will tell you to stay out of overdrive altogether. Some on this list use it when towing, but treat it with cotton wool, and change down at the slightest hill or head wind. There gearboxes are hanging together so far.
The problem is that earlier boxes were a 4 speed with a 5th tacked on the back and they failed prematurely. Todays boxes are designed better with 5th in the middle of the layshaft.
From a purely theoretical point of view, 4th gear is straight through the gearbox and minimal wear will occur, but 5th is using the layshaft and all the bearings on the mainshaft, layshaft and input shaft have maximum torque applied to them (@ approx 2000 rpm) so things start to heat up and if anything is going to kill the box it will be this type of condition.
Same goes for autos in o/d as the ratio is higher than a manuals.
Remember, to get x amount of torque to the rear wheels in 5th gear, 1.3 times x amount must be put in at the motor end of the box.
Andrewwheredayathinkwer mike?

Technological advancement is a funny thing. all the manufacturers want to improve what they offer but nine time out of ten they are just band aiding what they used the year before.
Most of the gearboxes were originally designed as four speed boxes and fifth gear has since been added with alterations in design and tooling but that does not change the fact that they were designed to run four gears. A lot of five gear boxes have the gear just , if you will , tacked on. The perfect example of this is the Hilux gearbox. You can tow as much as you like in fifth gear but when towing or at any other time for that matter fifth should be used as an overdrive gear in a no load situation, it is not meant to take huge torque loads, when you strike resistance such as inclines or strong winds use fourth and avoid a costly repair as it will last heaps longer.

Have been towing in 5th gear for most of the klms on my GQ TD42. I think it's done more klms towing than not! Obviously, I change down when I get to a hill, but not prematurely...
I have towed at various times two boats (one about 1.4 tonnes and the other probably close to 3 tonnes) and a caravan (about 2 tonnes laden). I always use 5th gear and gearbox is as good as the day it was built... (1991) I have never had a problem and never get any metal on the magnet when I change the G'box oil.
But then Nissan TD 42 gearboxes are built like a brick ****house.

Put a oil temp gague in the gear box, then you make up yr own mind. Do the same when comparing high and low range

Lets know, you will be surpised

Cheers Jol