Best tire pressure with 51 psi max Defender HIs

As explained on this Tire Rack reference page (https://www.tirerack.com/upgrade-garage/what-is-maximum-load-inflation-pressure-for-a-tire), the "MAX PSI" branded on a tire's sidewall indicates the maximum COLD inflation pressure as a reference. This in-turn carries relevance to the max load rating of the tire - https://www.tirerack.com/upgrade-garage/what-is-maximum-load-for-a-tire.

However, this number doesn't take into account the handling characteristics that your vehicle was designed with via its various engineering parameters, so filing your tires to this pressure can potentially cause some wonky behavior, especially at the limits (i.e. in true emergency situations, where we by-definition push the tires close to their 10/10ths).

This in-turn is where the vehicle's door-placard recommended pressures comes into play: however, this itself has the caveat of being valid only for the OE tire and tire sizing.

Going aftermarket can potentially cause things to change, here.

The sad truth is that while we all think that we can subjectively judge our tire's "feel" through our input behind the wheel, it actually takes A LOT more skill and knowledge to tease out such characteristics - skills which the manufacturers literally pay actual professional test drivers to assess. Unless one's driving record/CV/résumé as a professional or amateur actually reads of a similar caliber, one is just fooling themselves - and this is something that the experts at Tire Rack have said in so many words on NASIOC, where the crowd is tough and nobody minces words to save anyone's feelings. 😅

That said, practically speaking, inspecting tires for incongruous wear patterns as we change its cold inflation pressures can give a pretty good indication of whether the (specific aftermarket or OE) tires are being optimally inflated for our manner of driving. Here, it's important to not just "look" at your tires, but rather actually take some measurements using a tire tread depth gauge. While gross issues can usually be readily seen, particularly by experienced eyes, judging depth isn't something that can be done accurately in any way other than measuring (which is why tire techs actually use gauges!). Be aware that the tire will wear differently in different spots, so taking measurements in different locations around the tire is important: typically, inner/middle/outer measurements are taken. Similarly, remember that there's going to need to be some level of grace given to our skill to measure accurately as well as how "sensitive" the instrument itself may be: with gauges that measure in 32nd of an inch, understand the real-world (in?)significance of a 1/32nd difference, when measured in this way.

In terms of the overpressure warning, I've yet to see it on my Ascent, but I did once see it on my '16 Outback 2.5i Limited, as I sat in "car line" waiting for my daughter to be dismissed from summer camp, with one side of the car baking in the hot sun, on asphalt, while the other was in the shade afforded by the building. This is a tale that I've relayed many times both here as well as in the SubaruOutback.org community. IIRC, TPMS read the pressure on the sun-baked side somewhere north of 45 PSI. IIRC, there's at least one report on SF.org of someone seeing TPMS warn at 45 PSI.

I haven't run my current 245/60R18 Falken Wildpeak A/T Trail at 36 cold for long enough to really get a good feel, but at 37 cold, I would routinely see them at around 41-42 PSI via the dash TPMS readout, running on 80-90 deg. F., ambient, sunny days. I've never seen TPMS warn at such pressures, even after extended driving.

I tried to rip the max/min warn values from the ECU via my AUTEL TS508, but I couldn't remember how to do so (or even if it is possible). I'll check back here if I find a way. With TPMS tools like the TS508, I know that there's the ability to re-write the placard value (to correspond to out-sized changes in tire specs), but I haven't seen anything about reading or modifying the max/min warning values.

An interesting topic!

While searching got "truth" or at least some technical discussion on the topic, I came up with - well, not much 😱 The usual internet provided articles, however, there were a few consistent themes and statements from a variety of companies, organizations, forums, etc.

One can almost sense the authors of the many articles avoiding legal liability for their comments.

Paraphrasing a few:
1. Vehicle manufacturers try to pick a sweet spot for mpg and comfort
2. Vehicle manufacturer recommended pressures are minimum
3. Modern tires have lots of margin (but no clear numbers)
4. Higher pressures than posted on the car, to a point, improve mpg
5. Higher pressures result in less relative pressure increase with driving as the tires flex less, creating less heat, so more heavily loaded vehicles should have higher pressure
6. Too little pressure creates flex in the tires leading to potential failure with accelerated heating
7. More pressure could impact comfort (did not see any mention of damage to suspension, though)
8. Lower profile tires benefit from more pressure to help prevent damage from things like potholes and curbs
9. Many cited "sidewall minus 10%" as optimum
10. Found almost nothing on TPMS device standards, or when overpressure warning would occur (am assuming thus may be related to different implementation by vehicle OEMs?)
11. Overpressure relative to load results in more wear on center of tread face. No tests found that might look like this: take an Ascent, fill fuel tank, load to a percentage of max, then inflate tires until the form of the tread begins to curve causing more wear in center of face, then increasing load on vehicle until face of tread flattens to optimum. Leaving a lot of test points and analysis opportunity out of this very brief "what if"
12. Picking a pressure and driving until, over time, the wear pattern can be determined, is probably the only way know for sure - however, so many inconsistencies might affect this?

AscentForumUser said:

An interesting topic!

Click to expand...

Tires always are!

While searching got "truth" or at least some technical discussion on the topic, I came up with - well, not much 😱 The usual internet provided articles, however, there were a few consistent themes and statements from a variety of companies, organizations, forums, etc.

Click to expand...

Yup. Tire science is really, really interesting - Sport Compact Car dedicated a whole half-issue to write this up a couple of decades ago, and they conceded that they were barely scratching the surface. The amount of money these tire behemoths throw into their testing and development is just mind-boggling: I was given an insider glimpse into it some 12 years ago, and it just floored me.

In any case, let's continue this most interesting discussion.....

8. Lower profile tires benefit from more pressure to help prevent damage from things like potholes and curbs

Click to expand...

Back when Subarus were shod with Bridgestone RE92s (BL/BP Legacy, GD WRX), there were numerous reports of blow-outs related to sidewall damage occurring from slight overinflation (i.e. what hobbyists/enthusiasts would consider reasonable as a "corrective measure" to the perceived shortcomings of that OE tire/fitment) . Conventional wisdom seemed to fall apart, there...but yes, slight over-inflation has always been the "rule of thumb," in terms of helping low-profile tires with pothole/curb damage.

Just as any other such "rules," empirical evidence may really call it into question (more, below); yet, still, it's often a good place to start, if we can only keep from approaching it in a dogmatic manner.

9. Many cited "sidewall minus 10%" as optimum

Click to expand...

The other popular one is "sidewall minus 20%."

The problem I find with this is that it's an absolute based on the tire's load limit, but discounts any input that the vehicle manufacturer may have in terms of the vehicle's handling and performance (these in-turn translate into safety) characteristics.

However, it is much better at giving folks a reference point to start from when non-OE fitments come into the equation.

The most recent personal example I can give is the following. The max pressure branded on the Falken Wildpeak A/T Trail at 245/60R18 is 51 PSI. Filled to 37 cold and routinely checked, at just shy of one year of road-use (3-seasons, excludes winter, as I have a dedicated winter set), with approximately 3K miles on the tires, the tire shop measured tread depths of 10/32", 9/32", and 10/32" (out/mid/in; here I specifically cite shop [Tire Rack affiliated local NTB branch] measurements because I am purposely taking myself out of the equation), when I made a road-hazard claim (Falken AT Trails on 18" Wheels - what's your...).

How's either "rule of thumb" percentages working out with this one, versus empirical evidence?

10. Found almost nothing on TPMS device standards, or when overpressure warning would occur (am assuming thus may be related to different implementation by vehicle OEMs?)

Click to expand...

Same as my search from last night Into this morning. I think you're right, in that this ties into the vehicles parameters more, so it's just not there. I can't get my TS805 to pull this from my Subarus, and I don't see any mention of it in its general documentation, either, for other marques.

11. Overpressure relative to load results in more wear on center of tread face.

Click to expand...

Empirically, this has been my own experience over the last 30 or so years, with various vehicles, with direct tread depth measurements. However, whether not being blinded to the results, my own (lack of 😅 ) skill and other inconsistencies (what you'd cited in point #12) could potentially have caused bias, I honestly can't rule out.

And that's what I want to leave off with, this reply -

Many times, we conflate what we see/feel with what actually should be. How many times have we read here that folks want to over-inflate their winter tires so that they can reduce the "marshmallow feel" of their winter tires in corners? Isn't the actual purpose of winter tires to increase one's safety margins in wintry conditions, versus cutting sharp corners in the clear?🧐 Alternatively, we want to fight empirical evidence and hard data with the butt-dyno, like when Dave Buschur posted dyno runs after dyno runs years ago on NASIOC to disprove the supposed power enhancements of popular "Short Ram Intakes" (SRI), repeatedly showing that heat-soaking actually prompted the ECU to pull timing.... No-one wanted to actually see the data, because they wanted to believe that the extra induction noise they were hearing was actually the vehicle producing more power.🤯

As a benchtop scientist, I'd prefer that I take my feelings and beliefs out of technical conversations.

But I know that no matter how hard I try, it will always color such exchanges.

AscentForumUser said:

I had Discount Tire rep measure the Falkens at 35k just prior to replacing.

Click to expand...

So that's even better, then, as empirical evidence - it was an actual measurement taken by someone who supposedly knew what they were doing, versus a simple "look/see."

I had in my mind they were rated at 44 psi on the sidewall.

Click to expand...

Mine are on the bottom of my tire stack, and I'm too lazy to unstack this time of night, so....