Tire Pressure?

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TCM75

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When I bought my '15 LR4 I didn't check the pressures until driving around for about a month. Was astounded to find out the dealer set the pressures to about 50 front and rear. Drove well enough (stock Continentals) but I lowered them for better compliance over the bumps. 42 and 42 seemed to work well

When I put on the Duratracs I experimented with different pressures and have also found 42 and 42 are a good setup. I now have 28,000 miles on them and I have not needed to rotate as wear/tread depth is even across all tires. Works for my driving style and mix of driving of about 80% country roads and 20% highways.
 

ktm525

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Pavement: 37 to 37.5 front, 40 to 43.5 rears (cold, usually in the morning with whatever ambient temps).

(ask me why 0.5 increments if curious)

I could be wrong but I believe by "Cold" the manufactures actually mean the non-driven air temperatures inside the tires and not necessarily the morning cold temps. This is because the air inside the tires get hot as they are driven, hence the varying 2 to 3 or 4 PSI increase after having been driven for a while (this PSI range could also vary depending on the style of driving). And depending on the ambient temps it could take 3 to 4 or more hours for the air temps inside the tires to stabilize back to normal pre-driven temps.

So to make it simple, they just ask you to set the PSI levels in the morning as they know that by then its usually been several hours since the vehicle was driven last.

Yes all pressures should be COLD meaning roughly the ambient temperature of the expected temperature for the next few weeks and not driven for awhile (ie. overnight) and no sun beating down on one side (shade).
 

manoftaste

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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:


Front.JPG
Rear.JPG



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.
 
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cperez

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I guess only LR engineering can answer that question.

That was a good thought exercise with some solid reasoning. I should probably pay more attention to my tire pressures. At this point I may use that info to intentionally cause premature wear on my Contis. :D
 

mpinco

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Standard P-metric factory tires or LT / Load Range E? Makes a difference.

For your standard factory spec tire I would run recommended pressures except if loaded or towing.

For load range E I would inflate to higher pressures depending on tire and load, typically 50psi as the max pressure for them is 80psi.

Understanding Tire Load Ratings

".........The only things P-metric and LT tires have in common is that they’re black and round,” Dennis Franklin says. “LT fitments have more rubber: heavier plies, a deeper tread, and a stronger bead bundle. More rubber means greater weight or mass, which generates and retains more heat.”......"

If you underinflate a load range E tire it will run hotter, wear more and add stress. Because of their construction you should be running at higher inflation levels than factory spec.

I personally run load range E because the LR4 is heavy, it track better with stiffer sidewalls, tows better and I want sidewall ruggedness. When loaded for camping and towing with P-metric tires I could feel the sidewalls flex/roll.
 

ktm525

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Lawyers in the US. The LR rear spec assumes full load. I have noticed this with other European cars. We usead to get a half load and full load air pressure spec. Now? It seems the NA market gets the full load spec only. For me any more than 38 psi in rear results in premature centre wear.
 

iconoclast

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Remember when Ford/Firestone had all those issues? This is was the solution. All labels had to be for full load scenarios to avoid being underinflated. As we all know underinflated is worse than over so the regulators and powers that be decided that it would be best to assume full load at all times (or Americans are so fat that they achieve full load just by being in it) is the best solution to avoid blowouts.
 

ktm525

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In my case the ride of the LR4 is much better at "proper" spec than when having the tires inflated as if it was fully loaded (ie. towing).

For me the goal is ride/tread wear. I keep airing down until I get a nice consistent wear pattern across all four tires. In the odd scenario where I am fully loaded (7 people, and ski box and/or towing then I air up to 42 in the rear and leave front at 36.
 

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