Richard Moss wrote: Because the torque is all being passed through the reduction gears in the transfer box (AKA transfer case) then for a given vehicle speed you have a higher input shaft speed. That higher input shaft speed creates additional heat in the box and that could lead to over heating and potential damage to internal components

Yes, true. LO range limits the vehicle speed and that's where problems in LO happen - the driver feels it's going slow, so s/he keeps it floored and screams around at high revs.

But let's say comparing 1st HI at 30kph and 3rd LO at 30kph both at 3500rpm, there should be no issues of strain or overheating. In both cases the revs are 3500 so the engine is turning at the same level of strain, the gearbox is turning different diameter gears (1st and 3rd), and the transfer case is turning different diameter gears (LO and HI).

So instead of comparing ground speeds between HI and LO, it's better to compare revolution speed (rpm).

One point to add to Richard's caution about speed is that if you look at the diagram again, you'll see that the teeth on the HI gears are cut diagonally (helical), but the LO gears are straight cut (spur). Wikipedia explains that:

"...spur gears are used for low speed applications and those situations where noise control is not a problem, the use of helical gears is indicated when the application involves high speeds, large power transmission, or where noise abatement is important."
http://en.wikipedia.org/wiki/Gear_teeth#Helical

So by design, it is clear that LO gears are not for high speed use (speed calculated as rpm, not actual ground speed).

In other words: straight cut gears are VERY noisy compared to helical ones.

Sorry I've been away from the forum for a few days, guys, but I'm delighted with the replies I got here! Many thanks! They were very clear and informative.

Taher, to answer your question, I've been experimenting 4L regularly during my last few drives, and it's been extremely effective at moving me through the super-soft sand. I just switch to 4H when I'm making a run at a big tall thing and need to get up a good head of speed.

Richard Moss wrote:In other words: straight cut gears are VERY noisy compared to helical ones.

That's right - also they don't like sudden bursts of torque or change in rotation. The diagonal gear teeth mesh so that there is always maximum surface contact between the teeth, but spur gears initially only touch at the tips and the contact surface increases gradually until fully meshed, and then again the surface gradually decreases until they separate.

Cheers Jane when UAEoffroaders put their collective knowledge together, we are rich indeed! If you consider all the experience we each bring to this site, the amount of combined know-how regarding desert trips, the vehicles, and the terrain itself, we are a phenomenal source of knowledge 8)