I found this on an engineering site. I think the information about max sidewall pressures is very interesing. Explains why the Michelins have a 35 psi max.

<<CapriRacer (Mechanical)15 Aug 06 7:45
OK, here goes:

There are several tire standardizing organizations in the world. In the US, it is the Tire and Rim Association (TRA). In Europe, it's the European Tire and Rim Technical Organization (ETRTO). In Japan, it's the Japanese Automotive Tire Manufacturers Association.(JATMA). There are others but those are the major players.

These organizations not only set the standard dimensions for a tire size, they also set the load curve - the relationship between inflation pressure and maximum load carrying capacity. While each of these organizations has a different "formula" that is used for the load curve, they consult with each other and not only are the dimensions very close, but the load curves are too. The key point here is that there is some slight differences, and you can see the differences when you look up the load carrying capacities of tires with the same "size".

At this point I should mention that these organizations - and the standards that they set - are voluntary. A tire manufacturer does not have to follow the standards. But it just doesn't make sense not to, as all the vehicle manufacturers use the standards to determine what tire size and what inflation pressure to use on their vehicles and this information is listed on a label commonly called the vehicle placard.

Part of the load curve standard is a point at which the load curve stops. For TRA passenger car Standard Load tires (that's a lot of qualifiers, isn't it?), it is 35 psi. The equivalent ETRTO and JATMA based tires use 36 psi.

However, all the standardizing organizations agree that standard load passenger car tires can (and should) use higher inflation pressures for certain circumstances - like high speed driving. So ALL the standards say 44 psi or 51 psi is an optional maximum pressure. - with no increase in load and in some circumstances a reduction in load.

Please note: ETRTO and JATMA use metric units, and I'm only discussing this in English equivalents.

Now we come to the source of the problem. What is written on the sidewall of a tire is perscribed by a government regulation. It has been many years since I actually read the regulation (and I can't seem to find it now that I want to quote it.), but it says something like "maximum load capacity and inflation pressure."

Some think the regulation means the maximum load and the corresponding inflation pressure. This would look on the sidewall like: Maximum Load XXXX at YY. Michelin seems to be the only one with this interpretation.

Pretty much everyone else thinks the regulation means the maximum load and the maximum inflation pressure. This would look on the sidewall like: Maximum Load XXX. Maximum pressure ZZ.

For passenger car Standard Load tires, the maximum pressure listed on the sidewall will always be 44 psi or 51 psi, even though the pressure where the maximum load occurs is 35 or 36 psi. (with Michelin being the lone exception)

So in some respects, the 44 psi VW lists on its placard makes sense. They are using the maximum inflation pressure available, and I'll bet the circumstances that VW says this pressure is appropriate are fully loaded and Autobahn driving.

For those of you unfamiliar with German cars - and their placards - they list several pressures and the circumstances where they are appropriate. This is not the practice for US vehicle manufacturers - they list only one pressure.

Hope this helps.


jadcock (Civil/Environme)15 Aug 06 8:06
Fascinating. Is 44 psi the "standard" maximum inflation pressure for passenger car all season tires? The reason I ask is because our Grand Caravan specifies a recommended tire pressure of 36 psi front and rear. When we bought tires for it last year, we had no problem finding tires that would allow that pressure, and in fact the ones we bought bear a maximum pressure of 44 psi. But SOME tires out there prescribe a maximum pressure of 35 psi. Does this mean those tires don't necessarily adhere to the TRA? I wonder why, since most tires do seem to have a max pressure of 44 psi, some manufacturers would specifically limit some tires to only 35 psi. Does that necessarily mean it's a "weaker" tire in some regards?

CapriRacer (Mechanical)15 Aug 06 12:21
First, I'll bet the tires on your Caravan did not have the letter "P" in front of the size. This means the tires were either ETRTO or JATMA based standard. TRA tires start with the letter "P".

Second, 44 psi is common for Standard Load (as opposed to Extra Load) passenger all season tires, but there isn't a "standard".

I'll also bet the tires that you found with 35 psi on the sidewall were all Michelins.

And, no, if the tires have 35 psi written on the sidewall, they DO adhere to TRA. "They" have chosen to read the regulation in a certain way. But, and this is the confusing part, the tires are NOT weaker just because they say 35 psi. The use of 44 psi is still allowable for certain conditions and that doesn't change whether the sidewalls say 35, 36, or 44 psi

 

yeah i watched a video in school where a tire must pass certain stress tests before its deemed legal to be sold, and one of the test was how much psi a tire can take before it explodes. I wasn't sure what the tire was rated as but it did blew at 250 psi, so i highly doubt your tire will ever heat up to that moment if ever.

Although wwest is right about one thing, a hard compound can reduce grip but can enhance tire life if used properly.

I consider what's rated on the door is a "soft" compound, so best grip but at the expense of tire life. Medium would be around 40-45 psi and hard like me 50 psi + would be my guess.

 

That argument might be valid, on a presumptive basis, with the old Explorer, but I'm not at all satisfied that you can extend it to the Highlander. First, the vehicles are very different. The Exp was/is a relatively old fashioned body-on-frame SUV. Such vehicles inherently have a higher center of gravity than a unit body vehicle like the Highlander, and thus, are going to be naturally more susceptible to roll-overs.

Second, we really can't say anything about what the HH might or might do statistically, since we haven't seen any HH stats (at least I haven't...). We'd have to look to Toyota for such info, and we all know how a request for such would be met...

Third, suspension characteristics and design would have a large part to play in this question. If a vehicle allows too much roll in a sudden turn, actually, I could envision a harder-than-expected tire counteracting a tendency to roll over.

Anyway, I can see several other issues that would have to be examined before one could make a concrete conclusion one way or the other on this question. That said, I'd be reasonably comfortable adding a few psi over placard if I were driving a HH. But that's just me. Incidentally, I'm driving a TCH at the moment (not this exact moment of course!), and I've got the OEM Bridgestones a full ten (10) psi over the placard pressure of 32 psi (so they're at 42 cold, and the sidewall max is 51 psi). Though this sounds radical, it does not induce a hard ride as the TCH is already softly sprung, and the handling actually feels better, crisper than with the tires soft. Of course, a sedan is a different animal than an SUV.

Be reasonable and prudent, and you'll be OK.