how hard the motor is working, as you said on hills drop back a gear or if its a stong headwind and you are battleing to hold speed, any other time I have it in top gear.

Get a boost gauge to help you gauge your throttle level. Heavy throttle in a "high stress" part of the rev range (low revs) will blow it up, as with all turbo cars, it's bad to roll on accelerate outside of the power band

good comment by M &S about boost gauge

but 550kg ? fifth should be no probs at all unless there are probs with the car mechanically.

 5th on flat and change down on uphills like you said

Running in 4th will obviously give more revs, but I was told its the strain on the gears and associated mechanical parts which load up in 5th. Basically 4th gear is a 1:1 ratio on most manuals whereas 5th is higher, meaing the engine is less stressed but the gears are loaded up. 

The extra revs in 4th should actually lower the Exhaust Gas Temperatures compared to running in 5th and loaded up. More unburnt fuel getting into the turbo and heating it up.

I'm sure someone will chime in and prove me wrong.

FIWW

torque pro is my best friend when towing. any old crappy smart phone (doesn't even need a sim to run....except for download)

download torquepro ....$5 app
get a OBD2 Bluetooth ($10 ebay)...pair it up and away you go.

Everything your car knows and wont tell you at your fingertips.
I like it better than an overpriced scanguage as you can customise all your screens with whatever readouts and views you could possibly imagine.
can also analyse code faults and clear etc.

Don't leave home without it.

 D_d do you have a link for the OBD2

http://www.ebay.com.au/itm/ELM327-HH-OBD-Advanced-Bluetooth-V2-1-ODB2-Car-Auto-Diagnostic-Scanner-Tool-/331769112548?hash=item4d3efae7e4:g:7hQAAOSwT6pVzcsS

think this is what I have but think most will work just ebay search OBD2 blutooth

should work on any car that had the OBD2 port, years ago I had a fairlane '99 ?ish? didn't work on this even though it had a port but b4 my current car I had a STX navara and worked perfectly with that.

same Bluetooth conector works on my Colorado RG and Mondeo (both diesel)

 D_d_001

Nice info ... obd2 on order and torque pro downloaded....

also just a heads up : of course not all cars have all sensors.

Most very late models will have a huge array of stuff you can see only with something like torquepro.

my STX didn't have a gearbox fluid temp sensor (prob one of the most usefull sensors you could possibly have when towing )so couldn't display it on torquepro. but the Colorado rg does......but without the app, no way to see it. incredible !

I have to second D_d_001's thoughts on Torque Pro, got an old Samsung Galaxy S2 and the dongle sitting in the car (had it in an MN and MQ Triton, I have read codes out of a VE commodore) for this purpose. I spent ages fiddling around with all the screens and gauges to get what I was happy with though, there are so many options. It is a great app.

 Thanks,I've got the Colorado too,I tow my f/g CC to Exmouth every year,1200kgs.5th gear on the flats about 105 and change back to 4th for the hills?

 I've got the TD lux mate and when we go away up north we'll have about 500+kgs in the canopy and on the rack plus towing a 5.6m razerline on a tandem also with a few hundred kgs of gear in it.....

I always ensure i'm in 4th going up any rise or into any breeze and I sit on about 90-100km/hr....

Down hills or the flats I'll get into 5th....

Diesels love to rev and gearboxes dont like to be loaded, thats the theory i use and its worked for me on about a dozen trips thus far....

 In this case, it's more about the actual gearbox design. Five speeders of that generation were really a four speeder with another tacked on. 4th was 1;1, most direct route to get the power from the crank to the rear wheels. This has been done to death in the 4x4 forums,you can google it. Can't speak for 6 speeders, but the original question was about 5 speeders anyway. Don't get hung up on how many  revs more you are doing in 4th at the legal max of 100km/hr anyway--its probably the same rpm you'd be doing at 115 in 5th anyway, and I'll bet you don't get  concerned about that. Lower load on the engine at the lower road speed in 4th and right in the fat part of the torque curve,most likely.

I had a 2.8 diesel hilux from new,did a lot of relatively light towing( box trailer with camping gear and a dinghy rack) and i learnt early it hated being loaded in 5th. Fuel consumption went up, and the oil--well, diesel oil is always dirty ,but this was like tar after 5000k's of this, visible soot particles in the oil. Blowby must have been ferocious, and this on a relatively new car. . Went back to 4th and kept to the limit, very different outcome.

Later on bought a GQ TD42 Patrol ute, thought the extra cubes would mean I could tow in 5th. Much bigger load, and you could, but, strike a headwind and you were almost flat to keep 105 in 5th,billowing black smoke, and watching the fuel gauge drop like a stone. Back to 4th, 100k, no smoke and the fuel gauge stopped moving. No-brainer really.

Current towing vehicle is a GU TD42T ute--yes,"power" to do it in 5th, but all the same issues. Nowadays I'm towing 2 tonne of loaded boat and best part of 1.5 tonne on the tray. It uses 4th all the way to the Ningaloo station turnoff and back, 2650 rpm at 102 km/hr GPS--same revs as the 115 which is my normal, unloaded speed. Only time it will see 5th is on the way home, on the long downhill to the Galena bridge.

Like Timboon says,diesels prefer to be worked and gearboxes don't like to be loaded ( think of yourself trying to pedal a pushy uphill in a high gear) . 

 Just came back and so 4th is about 2600rpm at 100km/h, 5th is about 2500rpm at 110km so you are on the money ranmar, pretty much the same revs. Went up to 100km/h in 4th then change up to 5th and sat on 107 for most of the way and fuel consumption was only marginally more than when I'm not towing. 

I was just concerned after reading a lot about ruining the gearbox towing in 5th, stripping teeth etc...

DD001 thats a great idea the torque pro app.

Thanks for the help 

 There's a good app called forscan for fords, good diagnostic tool.

 If at all. I think it really only comes up when people do a LOT of towing of heavy stuff. Don't let fanboys from either Landcruiser OR Patrol camps tell you it only happens to the "other" types, they all can do it.

Below is copied from the Patrol forum.. the bit about the box being a four speeder with an overdrive applies equally to all older 5 speed 4wds, to the best of my knowledge. 99-2001 GU's had a known issue with a poorly designed 5th gear layshaft ( short splines with bad engagement), Nissan replaced all that failed under warranty.Towing in 5th bought it on quick. I'll bet mine never was because, when i bought it at 292,000 k from a deceased ( farmers) estate, it had zero log book dealer entries . So I certainly won't be towing in 5th.

 Quote  "This box isn't a 5 speed gearbox. It's a 4 speed with overdrive.

In 5th gear the torque travels down the input shaft and transfers across to the countershaft immediately then to the other end of the countershaft in the extension housing. There the 5th gear dog encapsulates the drive from the 5th gear countershaft returning the torque to the mainshaft and then directly to the transfer case.
The countershaft is totally and evenly laterally loaded.
The mainshaft is partially laterally loaded: outside the rear case in the extension housing (at the 5th dog) outside where it was designed as a 4 speed prior to being converted to 5 speed. Inside the input shaft/ mainshaft juncture where it is supported by needle roller bearings designed to support 1st - 3rd gear applications.
Bearings laterally loaded: input shaft; input shaft/ mainshaft needle roller; rear mainshaft; extension housing mainshaft; front countershaft; rear countershaft.

In 4th gear the torque travels down the input shaft and the 4th dog encloses that torque directly into the mainshaft inside the 4th gear dog at the input shaft/ mainshaft juncture and continues directly to the transfer case.
The countershaft idles at this point.
The mainshaft is not laterally loaded.
Bearings laterally loaded: None.

Can anyone else see the problem with loading up 5th gear yet?  "Unquote.

the topic of towing in 5th on a 4wd forum can end up like an E-tec thread on some boat forums 

 Seems as though Toyota diesels from the v8s onwards support Bluetooth obd2 devices..... Oh well at least I've got a new red light near my brake pedal