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Best tire pressure with 51 psi max Defender HIs

1468 Views 38 Replies 10 Participants Last post by  Packard8
Hi All,

I have always run my tires higher than car manufacturer recommendations on the door. Have always had good tire wear - and, likely better MPG than lower pressure. I ran the OEM provided Falkens at 36-38 psi - they looked really even across the tread and I could probably have gotten another 5K out of them when I changed at 35,000 miles two weeks ago.

The DIIs I just got mounted are 51 psi max. I will probably run them at 42-44 ambient after sitting overnight (time of check and fill).

What are this august group's thoughts?
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I've noticed the TPMS reading is usually 1-2 PSI below what 2 different digital gauges report. Is this typical?
I've noticed the TPMS reading is usually 1-2 PSI below what 2 different digital gauges report. Is this typical?
I think it depends upon the specific vehicle and specific set of TPMS. Most of mine have been accurate, but I've noticed lately that the TPMS on my MY19 Ascent is not quite as consistent as it used to be. Perhaps some of the sensors are starting to degrade after four+ years?
I think it depends upon the specific vehicle and specific set of TPMS. Most of mine have been accurate, but I've noticed lately that the TPMS on my MY19 Ascent is not quite as consistent as it used to be. Perhaps some of the sensors are starting to degrade after four+ years?
The sensors were purchased and installed by Discount Tire when I switched from 20" OEMs to 18" wheels and CC2s less than 2 years ago (I sold the OEM tires & wheels with the OEM sensors). The readings are consistent among the 4 tires, but just less than my gauges read. I go by the gauge reading, relying on the TPMS to warn of a leak or drastic change in PSI.
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I do sorta as @Packard8 - I go by the TPMS, normalizing that to my tire-fill gauges (because I don't know that they are actually accurate [lack of calibration/timely re-calibration], and as the under/over pressure trigger is what the TPMS sees and I can't change that [at least so far in as I know how, via my Auto TS805], and adjust based on what my tread depth gauges tell me (because unlike @AscentForumUser , I don't trust my eyes/hands nearly as much - nor my butt-dnyo, for that matter, LOL!). :)

For example, I know my main fill gauge reads ~2 PSI higher than all three of our Subaru's TPMS. I know that the gauge we keep in my daughter's vehicle reads spot-on versus her '19 Legacy's TPMS (and we also know that one of her winter setups experiences a very slow leak).

It's not like my pipettes at work, where I can self-check the calibration using just water. I mean, a microliter of water is a microliter of water......

Many tire shops will have a master gauge to which customers can calibrate to, but the problem with that is whether they've calibrated theirs. And how often will you return to re-calibrate? That's a lot of assumptions, right there. :ROFLMAO:

As long as the TPMS and my own gauges are consistently displaying values relative to each other, I honestly couldn't care less. That, combined with my own variance in technique simply means that in the real world, there's always going to be a bit of fudge-room. Besides, what's the plus/minus in any of these devices themselves? In terms of the TPMS installed in modern Subarus, supposedly the Schrader EZ-Sens(o)rs are +/-1 PSI, but I haven't been able to search up exact specs: Sensata (Schrader's OE) quotes many of their products at between 7 to 10 kPa, so +/-1 PSI would seem to be a reasonable assumption. 🤷‍♂️ Add to that production tolerances (i.e. between units, which is an unknown) and unique-unit issues (i.e. road use, weather exposure, variable battery degradation as @Jim_in_PA noted, etc.).... There's a whole host of issues at-play, where it comes the real-world readouts.

Regardless, it's not like I'm gauging in my mechanical hobbies, where I'm measuring in the thous using calibrated gauges. We all know about engines and even spark plug gapping, but there's other applications, too -


...or when I'm using specialized devices and software to measure in microns or nanogram concentrations at my day-job.

All that is to say that I'm pretty OCD, but this is one of those real-world things where I've learned to somewhat let-go.

The only time I've tossed one of my tire gauges is when it was providing inconsistently inconsistent readings. That's also the same kind of error for which I'd sent an optic back to the manufacturer for replacement.

And don't forget that as the tires experience different loads during the drive, they will also heat-up and cool-down differently, and that can cause 1 to 2 PSI differences simply between the four tires themselves, at any one time during your drive.

The reality of things is that it's the TIRE'S TREADWEAR that is our real-world readout. Here, we're typically doing three measurements across the width of the tire, with what is realistically a +/- 1/32" variability in our own measurements.

How off does one's tire pressures need to be, in order to see excess center versus shoulder wear? How does this relate to not only your chosen tire set pressures, but realistically, how rigorously you actually re-measure and adjust such pressures as the weather and your vehicle load and usage change?
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TSiWRX,

Good stuff - it IS interesting! 😎
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I really wish people would stop with this nonsense of "max sidewall pressure". Start with the pressure indicated on the door placard. If you know more than the engineers who built the car, feel free to adjust it to your specifications. Otherwise, it's another typical "I feel that"-type thing that permeates so much of the internet nonsense today.
Yeah, OK. But, didn't these door stickers previously show much lower pressures than now? And, even when these vehicle OEMs were providing higher pressure radial tires? And, weren't these lower pressures decided upon to provide softer rides? And, then tires were blowing up as folks weren't careful maintain that lower number?

I could go on, but ... my point was to search for some research and science that was shareable - and, consistent across OEMs, tire manufacturers, TPMS devices, computer management systems.

You sound a bit angry about this topic, DAC. Think of it as a discussion.
Yeah, OK. But, didn't these door stickers previously show much lower pressures than now? And, even when these vehicle OEMs were providing higher pressure radial tires? And, weren't these lower pressures decided upon to provide softer rides? And, then tires were blowing up as folks weren't careful maintain that lower number?
The Ascent? No, it was always the current numbers. I had an early '19.

Overall - as in historically? I honestly don't know. My time with autos doesn't extend back to bias ply.

"Softer" ride is a part of the equation, but by far not all, and it's probably not correct to say "softer" either. :p Part of how the suspension works has to have the tires figured into the equation. GM recommends 30 PSI for the C8 Corvette. Max inflation pressure as spec'ed by Michelin is also 51 PSI. Tire science is very, very complicated - the compliance offered by the carcass has much greater implications than just for ride comfort tuning. To-wit, look at what's happened in F1 in recent years.

In terms of tires "blowing up," many occurrences could cause such catastrophic damage - over-inflation can also cause similar issues either from the tire's reduced ability to resist road-damage or from excessive and abnormal wear. Here, remember what I alluded to of OE Bridgestone RE92s from the mid-oughts?

Finally, look at it this way -

How many reports of weird handling due to factory overinflation (for shipping, i.e. pre-delivery - as in that situation is supposed to be corrected before the vehicle has been delivered to the customer) on the Ascent - typically 40 PSI or greater - have there been on this Forum and the Ascent FB Group alone?

While slight overinflation is at times (i.e. prolonged highway driving, increasing hydroplane resistance, helping with transitional-weather cold-pressure-drop issues, etc.) beneficial (and higher pressures often used for actual racing or other higher-load situations), excess overinflation is definitely not (after the racing event, you'll want to air-down for the drive home). The flip also goes for dropping pressure, which helps with self-extrication or off-roading (i.e. sand, mud - and it is also known that after such events, you'll want to air back up).
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I was talking to the era where radials had been present for many years, but OEMs were still specifying lower pressures in spite of the advantages of higher pressures - well within tire manufacturer guidance.

My point is that religiously adhering to something provided by a manufacturer - even a Subaru (gasp) product - may not be optimum.

More data would be outstanding - and, of course, serve to extend the discussion :)
I was talking to the era where radials had been present for many years, but OEMs were still specifying lower pressures in spite of the advantages of higher pressures - well within tire manufacturer guidance.
Alas, that was well before my time, so I cannot speak to that with any authority - the limits of my knowledge applies to modern radials.

The only thing that I can say with respect to this evolution is just that - that it's an evolution. As we understand more about one technology and further refine it, there will be changes in our methods and practices. Today, automotive tire manufacturers spend untold amounts of cash to find suitable OE solutions to their needs to balance ride NVH (it's not about a "soft" ride, but ride characteristics that their target demographic finds acceptable or engaging), handling and performance (translation: safety - for example, braking numbers don't come from the vehicle's brakes alone), and also fuel-economy and tire longevity. While most of us hobbyists and enthusiasts may not agree with our vehicle manufacturer's selections and recommendations, the vast majority who fall under the bell curve are satisfied.

But towards this -

My point is that religiously adhering to something provided by a manufacturer - even a Subaru (gasp) product - may not be optimum.

More data would be outstanding - and, of course, serve to extend the discussion :)
I absolutely agree to both points, and I think that this discussion exemplifies that very thought: that dogma can and should be challenged.

After all, we have each seen by our own first-hand experience and empirical data that the dogma of "max pressure minus 10 (or 20) percent" really does not guaranty ideal real-world results: just as strict adherence to what's blazed on the door placard also likely does not, either.

And that's the trick, right?

That we actually have to have good evidence, and arrive at such new conclusions by having exercised proper logic.

It's always a fight to go against inbred conventional wisdom and institutional inertia. It's always a challenge to blaze against the accepted dogma and beliefs. Entering that fray, we'd better be prepared to prove our point, and having sound evidence is the starting point. Here, in this thread, some mistakes were made with initial assumptions, and the ensuing arguments shouldn't really have been arguments at all, as empirical evidence bore out. ;)
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Data to add -

My daughter and I finally swapped her winters (Michelin X-Ice, OE fitment for '19 Legacy 2.5i Limited) for her 3-seasons (Michelin CC2, at 235/50R18). We put things off by a week earlier this spring as there had been one more storm on the horizon: and after that, it was back-to-back-to-back weeks of various school and sports activities for her. 😅

In any case, as a final step in the process, we'd "true" her pressures after mounting - either letting out or adding air after winter storage. The CC2s and their rims being new, as you can imagine, at 75 deg. F. ambient, it was all about letting out what I'd typically pump in for storage (40 PSI, cold).

What really caught my attention is that at a couple of points in the process, I got spuriously high readings from two of her TMPS sensors - they read HIGHER after I'd physically let OUT air, than they read BEFORE I'd done so.

In each case, the spurious readings were in the tenths-of-PSI range.

This observation reinforces what Sensata cites of their TMPS accuracy being +/- 7 to 10 kPa.

As I stated previously how my view of TPMS aligned with @Packard8 's, that we see it less as an absolute, and rather utilize it more as a warning of drastic pressure changes, indicative of trouble at the wheel.
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Long story short. My TPMS alarm went off when tire pressure hit 51psi. Let air out at next rest area. So, when I tow, right or wrong, I'm comfortable now with my cold psi at 44 in the rear and 41 up front.
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Interesting that overpressure warning occurs at most common max sidewall?

Update from trip we've been on:
  • inflated to 38 in Florida at ambient, about 76F
  • driving 65-70, ambient 85F, 40 psi, about 28 mpg
  • driving 75-85, ambient 90F, two left side tires 41 psi, two right side tires 40 psi, about 26-26.5 mpg
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Interesting that overpressure warning occurs at most common max sidewall?
Maybe for the Ascent, but definitely not for the '16 Outback.

There's also other reports of TPMS tripping a high warning

Update from trip we've been on:
  • inflated to 38 in Florida at ambient, about 76F
  • driving 65-70, ambient 85F, 40 psi, about 28 mpg
  • driving 75-85, ambient 90F, two left side tires 41 psi, two right side tires 40 psi, about 26-26.5 mpg
Just remember that transient differences can be due to road conditions, your driving (i.e. differential loading of tires through turns, etc.), and even sunshine on one side of the car versus the other.

Also, again, Sensata cites of their TMPS accuracy being +/- 7 to 10 kPa, so be aware of that, when judging things like a 1 to 2 PSI difference in the readout.

Finally, aero issues can affect mileage at higher speeds. The Ascent isn't the slipperiest thing out there. 😅 While the tire's rolling resistance -which is also attributable to the specifics of that tire, versus another make/model of tire and is not a function of inflation differences alone- factors into the equation, things like wind resistance and the drivetrain's efficiency also exert tremendous impacts.
I'm not a "car guy" by any stretch, but I try to learn and do the right thing as best I can. Tire pressure is an area that's always confused me, for I've read different things in different places. Seems like the majority view, both on this site and other places, is to follow the numbers inside the door as per the manufacturer, but I've always had a bit of trouble with this not making logical sense in my head. Seems like 2 completely different tires made by 2 different companies with 2 very different max pressures printed on their sidewalls wouldn't or shouldn't always be inflated the same way. And the car manufacturer has no clue what new tires you'll be buying, so having the manufacturer generically overriding the company that made the tire seems a bit weird. Then again, the car company obviously knows their cars, and probably has a pretty good idea of what tires and pressures will work best with their cars (although the whole "softer ride" bias may have some merit, thus skewing what they know with what they want). Maybe what would work best is if the car companies gave a more detailed recommendation on tire pressure, with a broader range of numbers for if drivers prefer softer vs better mileage/wear/etc or whatever else. And maybe even including tire buying recommendations as far as how hard or soft a tire to even buy in the first place. Seems like buying a tire that says max 50 psi but only inflating to 32 is almost as "off" as buying a max 35psi tire and inflating it to 50 or more. I'd want to do what's best for the tire based on the tire manufacturer's knowledge, whilst at the same time do what's best for the car based on the car manufacturer's knowledge, and when those aren't in sync, esp when you're not a "car guy" then it's just confusing.
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I'm not a "car guy" by any stretch, but I try to learn and do the right thing as best I can. Tire pressure is an area that's always confused me, for I've read different things in different places. Seems like the majority view, both on this site and other places, is to follow the numbers inside the door as per the manufacturer, but I've always had a bit of trouble with this not making logical sense in my head. Seems like 2 completely different tires made by 2 different companies with 2 very different max pressures printed on their sidewalls wouldn't or shouldn't always be inflated the same way. And the car manufacturer has no clue what new tires you'll be buying, so having the manufacturer generically overriding the company that made the tire seems a bit weird. Then again, the car company obviously knows their cars, and probably has a pretty good idea of what tires and pressures will work best with their cars (although the whole "softer ride" bias may have some merit, thus skewing what they know with what they want). Maybe what would work best is if the car companies gave a more detailed recommendation on tire pressure, with a broader range of numbers for if drivers prefer softer vs better mileage/wear/etc or whatever else. And maybe even including tire buying recommendations as far as how hard or soft a tire to even buy in the first place. Seems like buying a tire that says max 50 psi but only inflating to 32 is almost as "off" as buying a max 35psi tire and inflating it to 50 or more. I'd want to do what's best for the tire based on the tire manufacturer's knowledge, whilst at the same time do what's best for the car based on the car manufacturer's knowledge, and when those aren't in sync, esp when you're not a "car guy" then it's just confusing.
I've always thought the max pressure is directly related to the load rating, or max weight. This is from experience mostly with truck and trailer tires. Let's say your truck has a max weight (with cargo) of 10K lbs. You want 4 tires that are rated for at least 2500 lbs each. You look for tires with a 2500+lb rating. That rating is assuming max PSI (50PSI on the sidewall). If you mount these same tires on a truck that has a max of 5K lbs, you don't need to air the tires to 50 PSI.

Maybe this is an oversimplification, but when looking at tire specs, the max weight is shown with a PSI inflation number, usually the max on the sidewall.
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I'm not a "car guy" by any stretch, but I try to learn and do the right thing as best I can. Tire pressure is an area that's always confused me, for I've read different things in different places. Seems like the majority view, both on this site and other places, is to follow the numbers inside the door as per the manufacturer, but I've always had a bit of trouble with this not making logical sense in my head. Seems like 2 completely different tires made by 2 different companies with 2 very different max pressures printed on their sidewalls wouldn't or shouldn't always be inflated the same way. And the car manufacturer has no clue what new tires you'll be buying, so having the manufacturer generically overriding the company that made the tire seems a bit weird. Then again, the car company obviously knows their cars, and probably has a pretty good idea of what tires and pressures will work best with their cars (although the whole "softer ride" bias may have some merit, thus skewing what they know with what they want). Maybe what would work best is if the car companies gave a more detailed recommendation on tire pressure, with a broader range of numbers for if drivers prefer softer vs better mileage/wear/etc or whatever else. And maybe even including tire buying recommendations as far as how hard or soft a tire to even buy in the first place. Seems like buying a tire that says max 50 psi but only inflating to 32 is almost as "off" as buying a max 35psi tire and inflating it to 50 or more. I'd want to do what's best for the tire based on the tire manufacturer's knowledge, whilst at the same time do what's best for the car based on the car manufacturer's knowledge, and when those aren't in sync, esp when you're not a "car guy" then it's just confusing.
You say you're not a "car guy," but you're absolutely correct in your reasoning on the tires -


That's a lot of verbiage, but focus on the first and third paragraphs (unfortunately also the longest paragraphs, but hey, it narrows things down by almost half! 😅 ).

So, see? You were right-on, in your reasoning. :geek:(y) You may think you're not a car-nerd, but you are!:giggle:

The thing to remember is that the empirical readout in the real-world is tire wear. If you rotate your tires or fit seasonal sets by yourself, that's a great opportunity -and set intervals- to observe tire-wear and compensate as-necessary. If you favor going to a shop, the tech or inspection report should tell you the same, but if they don't, don't be afraid to ask.
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I really wish people would stop with this nonsense of "max sidewall pressure". Start with the pressure indicated on the door placard. If you know more than the engineers who built the car, feel free to adjust it to your specifications. Otherwise, it's another typical "I feel that"-type thing that permeates so much of the internet nonsense today.
Neither is correct. Load rating vs weight plus safety margin (eg: an FoS of 1.3) is how to calculate it.

Here's a worksheet Falken and I were going through when we were figuring the proper inflation for the Wildpeak AT Trails and AT3W's for the Ascent.

I've highlighted two columns. Note how the load index (left purple column) changes the load capacity (right purple column) at the SAME inflation.




Falken and I worked out a number of different combinations of load ratings and pressures, at various sizes.

ANY reputable tire company can calculate the proper inflation based off using 1,648 pounds plus an FoS of 1.2 or 1.3, based on their tire's characteristics. But, using the door placard info is wrong, unless the tire is the same load rating and size (plus 1-2 psi for all terrains to deal with the mushiness handling feel).
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Neither is correct. Load rating vs weight plus safety margin (eg: an FoS of 1.3) is how to calculate it.

Here's a worksheet Falken and I were going through when we were figuring the proper inflation for the Wildpeak AT Trails and AT3W's for the Ascent.

I've highlighted two columns. Note how the load index (left purple column) changes the load capacity (right purple column) at the SAME inflation.

View attachment 21168


Falken and I worked out a number of different combinations of load ratings and pressures, at various sizes.

ANY reputable tire company can calculate the proper inflation based off using 1,648 pounds plus an FoS of 1.2 or 1.3, based on their tire's characteristics. But, using the door placard info is wrong, unless the tire is the same load rating and size (plus 1-2 psi for all terrains to deal with the mushiness handling feel).
Note how the load index (left purple column) changes the load capacity (right purple column) at the SAME inflation.
Makes sense, as the same size tire's weight rating will change with the tire's internal construction (8PR vs 10PR vs 12PR, etc), at the same PSI of inflation. I have some 14" bias ply trailer tires rated for 3400 lbs @ 80 PSI (12PR).
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