Using 5th gear around town - any damage done?

Hi Mike, your advice from your friend is generally true, a gearbox is in effect a torque multiplier, by that I mean, that in 4th gear of most manual gearboxes, it is 1:1 ratio, meaning that available engine torgue at the flywheel goes straight thru the gearbox to the transfer case/tailshaft/diff/s. Think of the gearbox as having an input shaft (front) and a output shaft (rear), and the 5 gears allow a change in speeds between the input and output shafts, except 4th gear which is usually 1:1. When you go down thru the gears, the gearing change in effect multiplies the engine torgue, ie more engine crankshaft revolutions for given tailshaft revolutions, the input shaft is spinning faster than the output shaft, so in effect more energy is delivered to the output shaft, as you go down the gears. However, with 5th gear, it is an overdrive, the input shaft is spinning less than the output shaft, so is reducing the available torgue to the tailshaft.This is fine when cruising with a light/ medium engine load, and the engine producing near maximum torgue, which is generally from 2,000 to 3,000 rpm, depending on engine type, petrol/diesel, and design. In 5th gear, because of the overgearing, there is an energy loss, and low speeds in 5th gear, generally labour the engine, and allow excess torgue loss in the gearbox.
Ever see the old bloke chugging along the highway in a diesel 4wd, with a van or trailer at 60-80 km an hour with smoke billowing from the exhaust, well, thats him in 5th gear, labouring away. Would be more efficient and friendly to his vehicle in 4th gear, where the engine would generate sufficient torgue to allow a light throttle, and not have the high torgue load/loss happening as the power transferred thru the overdrive gearing in his gearbox.
In a nutshell, if the load is light, no trailer etc, flat terrain, 60-80 km/h is generally not a problem in 5th gear, but I doubt that any big efficiencies are gained. Hopefully others may have some input too, cheers.

thanks coghead, always used to wonder about that myself

I bought a 1985 Pajero (120,000k at the time)from a guy that used it to tow a boat around and he always towed in 5th gear. Just after I bought it I had to rebuild the box because the 5th gear cog and bearings were shot.

Mike, I had an NH Pajero which had 35k and no tow bar when I bought it. I regularly used 5th gear around town, and when towing (being very careful not to overload it). At 120k the gearbox was overhauled because 5th gear was shot (which cost a lot more to repair than you save in fuel costs by using 5th around town). The Pajero gearbox has 5th gear splined onto the countershaft. The splines were flogged out by low speed use. I now have a GU Patrol, which I bought new, and use overdrive only for speeds over 80km/h.

If I might add to the tales of woe. I had a MH Triton V6 that I purchased at 80000 k. The previous owner had a mini excavator that he towed around in 5th gear. I had to overhaul the gearbox at 110000k and replace 5th gear.By the way v6 Tritons have a very heavy duty gearbox Mitz Canter light truck I believe. Regards Ken.

Dear me, nice to see the mounting comments from Mitsubishi owners about their g/box problems at 120 to 160k, I have personal friends that have had the same, their appears to have been a case hardening problem with Mitsubishi g/boxes a while ago.
But I am sure other brands have had the occasional failure due to reasons, I remember the Nissan diesel 720's were like that, they would shake the box to bits, even for a thing that couldn't pull the skin off a rice custard..oops.. offended a few Datsun diehards now..
Some comment from current or ex Mitsubishi mechanics, or gearbox specialists would be interesting.