Thats just the reason. Its not about the engine, but more the gearbox. The powerfull engine puts a lot of strain on the overdrive gear when towing. Better to tow in a gear where the ratio is 1:1.
Do a search on the topics. This topic has been extensively discussed.
Fabba welcome to the forum
Try to get the gear ratios for your vehicle, whether online or in the car's booklet. The ideal is to tow in the gear closest to a 1:1 ratio as it puts the least strain on your gearbox. It is almost like a direct coupling between the engine and axle. In the Fortuner's booklet they specifically advise you to tow in 4th gear with the manual version, while it in fact has 5 gears.
One may easily argue what the heck, it will be the next guy's problem. However you never know. The gearbox might break on your watch or circumstances may force you to keep the vehicle longer than anticipated. That is why I stick to the best diesel I can find and try to put the least strain on the gearbox. This is also why I detest metro police roadblocks while towing. Those guys are not going to replace my clutch kit when it gets worn. All those stop starts with a heavy caravan are not good at all.
Welcome Fabian. I also used to tow in 5th or 6th gear. What I did was while in that gear I excelerated a little by just pressing down on the pedal. If the tow combination did not pick up a little speed I would rather change one gear down.
This resulted in using those gears mostly on level roads and on slightlydown hills. Currently I have an Auto box and found that the vehicle itself regularly selects 6th gear while towing.
Leon, ek het ook 'n outo en sleep maar in 5de. Ek kyk wel dat die engine nie bokant 2000 rpm hardloop nie. My vorige sleepding se ratkas het opgepak, so glo ek, omdat ek heeltyd net in hoogste rat gesleep het.
Fabba, the main thing about towing is to constantly look at your rev counter instead of the speedo. If your vehicle's highets torque is at say for instance at 2500 RPM's, try to keep it at highest torque RPM's without labouring your engine. If the engine is starting to labour, rather go to 4th gear.
I myself like to tow in 5th gear, but if my load is a bit heavy, I rather stay in 4th until I reach a downhill or a fair straight then I will select 5th again, but when facing an uphill, I prepare myself to shift down to 4th again to keep my constant speed of 100-110 km/h at highest torque.
As you mentioned that you have done some mod's to your exhaust and also chipped your vehicle, I done the same to my exhaust system, which gave me much more performance than other vehicles in my vehicle's class, but I rather play safe, because everything at the end of the day cost money and that money to spend on repairs, which could be avoided is another camping trip down the drain
Just my 2cents.
In addition to the above - judging by your avitar photo (3 litre towing a lightish low profile caravan), you should be able to tow that caravan without straining the gearbox on selected parts of the road. Just take note what the others what are towing with and what they are towing and you should realise you should have a comfortable tow in any gear whilst still taking "towing" into consideration.
I also selectively tow in 6th, but drop gears quite early on a hint of a gradient, just to play safe.
My concern is that the chipping adds power and you may not realise the additional strain on now imposed on the gearbox.
Fabba dis jou voertuig jy kan in enige rat trek,maar luister maar na die manne met die ondervindind, ek trek ook in vierde as ek n lang afdraande kry gaan ek 5de toe maar weer terug na 4de
Kon jy tussen al die teenstrydighede 'n ingeligte besluit neem Fabba?
Als kom maar met ondervinding en mens probeer maar dinge toepas om die sleep en lewe vir jouself gemaklik te maak en om die wat jy het te bewaar sodat jy dit nog lank kan gebruik.
Hier in Gauteng kan ons weer so ietsie leer by jou met die opdraendes en afdraendes.
Basically, I think Leon sums it up best. If you tap the accelerator and the vehicle surges, then fine. If not, drop a gear.
Remember down here in the Cape there are very few camping spots reached on level roads, there is always a mountain pass and steep hills to negotiate, so as long as the motor is not straining you should be OK. Those are good towing figures you have though...