Here are my thoughts about the "factory" recommended PSI levels.
I understand that the factory numbers are very important and that the truck has been tested with these numbers by the engineers for the optimum balance between the ride quality, gas milage and safety, etc. But I also know that the marketing/bean counters have lots, and I mean lots of interference inside the engineering department, to the point that sometimes this interference compromises the integrity and quality of engineering decisions. And I believe the latter may be the case in LR4' specified PSI levels as well. And here is my reasoning:
My 2006 LR3 HSE (with 19 inch wheels, full size spare and rear locker) came with the factory specs as follows:
33 Front, 36 Rear (Normal load), and 42 Rear (Maximum load).
You can see that in this case, the engineers had specified two separate PSI levels for the rear tires. And that made sense given the fact that the contact patch would change in size with the varying amount of weight on the rear axle. So for LR3, a truck with near 50/50 Front-to-Rear weight distribution ratio, specified 36 PSI for the rear tires under normal load conditions perfectly matched the specified 33 PSI for the fronts.
And under normal load conditions, 42 PSI in the rear tires would have been too much air, reduced ride quality, perhaps even unsafe, and would have allowed for the unwanted varying traction/road grip between front and rear axles, due to the reduced contact patch of the rear tires. And this difference in traction levels may not have been good for the near 50/50 front to rear driveline torque split either.
So in case of LR3, the engineers seemingly made the right call of giving us the two PSI specs for the rear tires. And I remember keeping my fronts at 33 and rears at 36 unless I was carrying lots of cargo, people or stuff.
Now when I got my 2013 LR4 (also with the 19 inch wheels, full size spare, and rear locker), the factory recommended specs went up a bit higher as follows:
36 Front, 42 Rear.
First thing I noticed there was that this time there were no two separate PSI levels for the rear tires.
For the fronts, the 3 PSI increase from 33 to 36 PSI made sense as the new engine may have gotten a little heavier going from 4.4 v8 to 5.0 v8, and maybe there were other extra or new components in the bay, and a revised ZF making the truck possibly a bit heavier in the front, hence the increase to 36 PSI in the front.
But what did not make sense to me was this jump of 6 PSI levels for the rear tires, from 36 PSI (normal load) in LR3 to 42 PSI in LR4.
Knowing everything that we know about these trucks, going from LR3 to LR4, the front-to-rear weight distribution ratio has generally remained the same and the rear axle has not seen that much increase of extra weight on it that it would actually require an increase of 6 PSI, from 36 to 42 PSI, for normal load conditions.
The only explanation I could think of is that the decision of single PSI level spec for the rear tires may very well be one of those marketing decision on LR' part to make it less confusing and "easier" for us, the soccer moms. And of course, out of the two specs, they chose the higher number of 42 PSI so the soccer moms are covered for both normal load and fully loaded configurations, and so they, the manufacturer, too are covered as far as any liabilities which may arise if the rear tires are inflated to a lower and the normal load PSI level spec while the truck is being driven loaded up to its maximum upper limit.
And they continued this tradition to 2016 MY as my current '16 (19 inch wheels, full size spare, and rear locker) have the specs of 37 Front and a single PSI spec of 43 for the Rear.
I am guessing that the 1 PSI increase going from my '13 to '16 on both the front and rear is due to the added weight of the now standard equipment factory running boards and/or to make the truck feel a bit peppier due to the reduced HP and torque of the SCV6 compared to '13 NAV8.
My driveway got coated with some dust last night due to it being windy, and the tread section of the tires picked some up, clearly showing the difference in the width of the contact patch. With 37 up front and 43 in the rears, you could clearly see the difference in the following pics. The tread on the fronts picked up the dust edge to edge but thats not the case on the rears, indicating that the rear contact patch is not really flat:
I mean, shouldn't contact patches be of the same size on all four corners under normal load/driving conditions, specially for a truck with AWD and near 50/50 front to rear weight ratio?
I have noticed the above difference in the width of the contact patches pretty clearly on my stock contis, WRG3, and now the TerrainContacts, all 255/55/19, whenever i got some dust in my driveway area. And I tend to keep a check on the alignment, etc.
I have now been thinking about setting my rears to 40 instead of keeping them at 43 PSI.
But I wonder that when LR made the switch from two separate PSI specs for the rear tires to just one, knowing that this would result into the size of the contact patch of the rear tires being smaller than the fronts, that if they have also made any adjustments to rear suspension and the front to rear torque split and/or the traction monitoring algorithms, etc. in order to accommodate for the difference in the traction/grip levels caused by the difference of 6 PSI between fronts and the rear tires.
I guess only LR engineering can answer that question.