4th or 5th Gear - again

Hi Ups and Downs

It's just the total weight. Labouring in 5th gear won't be good for the gearbox or the fuel consumption. Some years ago a friend with a nissan ute blew the 5th gear twice when towing a caravan. that was a manual box. Both time it was replaced under warranty, then Nissan issued an instruction not to tow in 5th.

In an auto the same principle applies. However with the auto everything may seem OK except when the torque converter is not locked-up a lot of heat is being generated which can cause serious damage. So by towing in a lower gear everything runs freer with less heat being generated. There has been a lot of discussion about this subject on the Landcruiser owners website.

Cheers
Racey

Ups and Downs - Once you're towing a trailer, the gross weight the engine is pulling is substantially increased. The load on the entire drivetrain is increased dramatically.

In direct drive (4th gear or top gear in older vehicles without OD), the torque from the engine is being taken in a straight line through the transmission - or in some odd cases, if through a gearset, then the gears are evenly sized (i.e., a 40 tooth gear is driving an identical-diameter 40 tooth gear).
Thus the torque loading on both gears (and gear teeth) is even.
In the case of straight-through drive, as in 4th gear with most 5 speed OD transmissions, the drive is simply the input shaft (clutch shaft) and output shaft (mainshaft) locked together by evenly-sized splined teeth.

An overdrive gear uses an indirect gear set, whereby a larger gear is driving a smaller gear.
In the case of say, a .8 ratio overdrive, the drive gear is 20% bigger in diameter than the driven gear.

In the case where maximum engine torque is being transmitted through the transmission - and the vehicle is heavily loaded with a trailer and approaching gross maximum combination mass (GCM) - then the teeth on the smaller diameter output (driven) gear are being stressed considerably more, than the teeth on the larger (drive) gear.

As a result, with constant use of OD in this situation, you stand a good chance of gear tooth failure on the smaller diameter (driven) gear. By changing back to 4th you remove the highly-stressed OD driven gear from use, and revert to a direct drive coupling or ratio, where there is not the same extreme gear tooth loading in the transmission.

In lower gears, a smaller diameter gear drives a larger gear, and the loading is on the smaller driving gear (and its teeth).
Thus, in an extreme transmission load, the failure is going to be in the smaller gear. However, this type of failure is far less common, because the smaller gear is designed to take heavy tooth loading.

In the case of OD gears, the gear teeth loading on the driven gear increases rapidly and substantially, as high torque levels are encountered.

I can speak from personal experience here. I used to own a 1968 model International tandem drive tip truck in the early-to-mid 1970's. This truck had a 6 cyl Perkins driving through a 5 speed main transmission and a 3 speed auxiliary or "joey" transmission.

The 3 speed "joey" consisted of underdrive, direct drive and overdrive gear. International warned about avoiding using OD gear in the "joey", in combination with lower gears in the main transmission.
OD in the joey was always the last gear you changed up to, and the first gear to change down as soon as the engine started labouring.

I got a job hauling sand to backfill a mine. The job involved backing up a 10M high ramp to tip the load. It was a pretty steep ramp, and being young and keen, I was always "speed oriented". I was on contract, of course!
I found that I could get more speed up, and climb the ramp faster, by using OD on the "joey" in combination with reverse in the main transmission.
The truck was always overloaded, because I was on mine roads, and axle loading limits didn't apply.

I did this for about 6 mths, before I heard an almighty BANG! one day, as I was reversing up the ramp, fully loaded.
The truck stopped driving. It was obvious the drivetrain failure was in the "joey" box.

I pulled the "joey" box out and upon dis-assembly, I found the majority of the gear teeth on the smaller OD (output) gear, completely stripped.
Thus the reason for the warning from International about the proper use of OD gear was driven home to me, in the most forceful manner!

Cheers - Ron.

Hullo Paul

A lot of good advice here.

What has not been addressed so far is your query about the difference in the weight being in the trailer vs in the vehicle.

There are 2 factors at play here. Given the same overall weight, the more the number of axles, the greater the rolling resistance - and hence strain on the drive train. Secondly, if you are towing a trailer or van, the greater the speed, the more drag is produced - hence strain on the drive train. This second factor also applies to modifying the shape of the vehicle, for example, by adding a roof rack and then by the frontal area and overall shape of what you put on the rack.

WRT the use of, in my case, 3rd instead of O/D, the fuel consumption is about the same and the stress is less and the gearbox (auto) temps lower.

Cheers
Andrew

Maybe once upon a time it wasn't advisable, may people these days tow in an overdrive gear and don't have a problem.

Sure the overdrive gear is smaller and the torque is multiplied in that gear, so towing in overdrive under extreme load is not advisable but along a flat or slightly undulating road should not cause any issues.

For every one person who thinks it's not OK there are thirty others who don't see a problem and have towed like that for years.

It's funny how you say people think it's OK to drive in 5th gear with a ton of weight in the back but put a trailer on weighing a ton and towing in 5th becomes a no no.....

We have towed in 5th gear for years and never had a gearbox failure.

If your vehicle pulls OK in 5th gear travelling in 5th but if it is labouring drop back to 4th.

Humans are a funny creature ...... we will always blame something for causing a failure but 9 times out of 10 we have no real data to back the claim...

yep!

4th gear (in most gearboxes) is the strongest because the gearbox input shaft is connected directly to the output shaft. There are no cogs involved.

Fifth is the often the weakest (or, more likely to fail first) because it is seeing the most load - aero, drag etc - and therefore doing the most work at highway speeds

Towing introduces even higher aero and drag loads - possibly as much as double - which is why you use more fuel when towing than with the same weight within the vehicle

HTH

The simple answer is to follow the vehicle manufacturers recommendations in the manual.
If the manufacturer tells you not to tow in 5th gear, then don't do it. If they don't mention it, you can take heart in the fact that they designed the transmission to tow in 5th OD gear at FACTORY RATED loads.

The greatest problems stems from people asking too much of engines and drivetrains.
They chip the motor, install a turbo, or overload (or do all three) - then complain when something breaks!

Overall, I've found that Jap transmissions have less margin for overloads and abuse, and are much more finely engineered, with tighter tolerances, than most American transmissions.

The Yanks build trannys with massive reserve margins of strength, and with the ability to handle trash in the oil much better. The Jap transmissions can't tolerate any water or metal debris in the oil, because they use such tight tolerances and relatively lightweight bearings.

How longs a piece of string? It depends on which model,which gearbox,what kind of terrain ,how much is in the trailer.
Other things such as the diameter of the tyres compared to the factory ones.
Too many variables for a straight yes/no answer

Here's my take on this discussion. I tow in 4th gear in my 80 Series manual. Why?
Two reasons.
First, it was recommended to tow in 4th in my owners manual for my first 80 Series back in 1993. I can't find the same advice in my 1997 80 Series owners manual but figured I would stick to the original instruction.

Second. Having fitted an aftermarket turbo, snorkel & 2.5" exhaust I also fitted an EGT gauge. The gauge tells me that to tow in fifth even on reasonably flat roads causes the EGT to rise to unacceptable temperatures (450 Degrees C and above). When towing in fourth at the same speed but at higher revs the temperature remains around the 350 C. That tells me that in fifth the engine is struggling, so 4th gear it is for me.

Its that simple. I'm trying to give my engine an easier life.

WBS

Hi Paul,

I agree with most here and will only tow in 4th (direct drive) but cant find anywhere a definitive rule on where the cutoff point is If I carry 250 kgs, 500 kgs 750kgs, should I use 4th gear? Ive been using the If it labours then use 4th rule and its worked on my previous 4wd's.

But now Ive got the hilux, I'm worrying more (cause the hilux clutch is crap) Sorry to highjack your thread but I'd like to know.

Cheers Wilko

Many years ago I was talking to a camper trailer manufacturer. He travelled all over Aus displaying his campers at shows, he always had 3 with him.

He carried one on the tray of his GQ cab chassis and towed the second one with the third upside down on top.

I had a GQ at the time and asked him about towing. He always used the full box. Would tow in 5th but as soon as the car started to slow he would change down to 4th. Usually at about 110km/h.

Now the GQ did have a truck gearbox but his advice has proven to be very good. That was 16 years ago. I have had the GU for the past 10 years and drive it the same way I did the GQ. I tow in 5th but the slightest drop in speed sees me shifting down to 4th. I don't have any fancy gauges to tell me what the engine is doing but my fuel figures tell a story. I can see when I have been lazy and sat in 5th all the way rather than shifting down.

With a 4.2 TD the temp gauge will also tell you when you are pushing it. On the Newell Highway between Coonabarabran and Narrabri sitting at the then posted speed of 110km/h in 5th the temp was slowly rising. In 4th at the same speed the temp dropped back to normal. I would never have thought that road at that speed would be a problem, but it was.

The GU has 200k on the clock and lots of that was done with about 1 to 1.1 tonne of trailer behind it. No problems whatsoever. But I listen to my car and don't stress it and I maintain my car. I reckon there is another 200k left in it and I will continue to tow in 5th.

I agree with those who say follow the manufactureirs advice and I can't mount a technical argument to oppose the mechanincs on this forum, but my experience is valid and is proven over many years. When my mates box stripped 5th I became concerned about mine. So I asked the head mechanic at the Nissan dealership. He looked at my VIN and engine numbers and said I would be Ok. On the Nissan there was a very clearly defined period which displayed problems, mine was built outside that period.

I think this is another argument that will continue as long as there are more than one person in the discussion.

Duncs

ahhhhh my head and eyes are hirting after all this

Maybe my choice of a big truck has solved all these issues???

Geoff

I cannot understand all the kerfuffle. It is sooooo simple. It is not a blanket statement that at 100KPH you should be in "X" gear.

Use the gear that does not "labour" the motor and doesn't need maximum revs that make it pop valves nor minimum revs that would shame any corrugated road. Then the motor is not stressed and all the drive train should be well within comfortable working parameters.

Comfortable for us (4.2td) is just about 1/2 to 3/4 revs with just enough accelerator pressure to retain speed, up hill, down dale, towing and not towing, bitumen and dirt, rocks and sand etc etc etc. Depending of course on RTA laws, road and traffic conditions.

I do not believe that anyone here is stupid enough to travel at maximum valve popping revs or at minimum kangaroo hopping revs. If you have any sense at all you will know when the combination is right for your car. It's not rocket science.

My 4.2td Patrol loves 2400rpm and I tow in 4th only as it is the suspect 5th gear model 2000.
I have tried the 5th gear but with over 2 ton behind at the slightest rise it will need to be changed down and by the time you change down then 3rd is required, hence I always tow in 4th and hardly ever need to change down.

I also did a fuel comparison on the flat roads and found that 4th used only .5 to 1 ltr /100k more so another reason to stick to 4th.

The old banger now has 250k on the clock and still runs good as new.

And yes I use cruise as much as I can.. Ease of driving for Me and the vehicle.

Those old 4.2 engines just keep going on and on.

Thanks to all who contributed and I now know that towing increases the stresses on the drivetrain due to wind and rolling resistance, rather than just because of the weight involved.

After 360,00 trouble free kilometres I must be doing something right, or just lucky!

Some really useful background to understanding the situation as well as some worthwhile opinions presented.

I will continue to tow in 4th gear with the 4.2 litre manual diesel having a 5 speed gearbox.

When just having a full load in the vehicle I'll do as usual in 5th gear, of course changing down to prevent labouring.

Thanks again to all.

Paul