I don't notice any compromise in handling at 29psi.

Why not? It's not a significant decrease and it works. It's 3 years for me at 29psi on the same set of Kumhos.


You are now being too careful and walking in eggshells [emoji16] Analysis paralysis..

Of course the tyre sizes can be played with within reasonable safety considerations.

For me, less tyre pressures have worked, and I can say long term.

Agreed on the minor change in psi.

Studies show that before tpms the majority of tires were under inflated more than 5psi though there is a chance some accidents were causes from it, the legislation that brought about TPMS requirement was based mostly on saving gas.

People that track their cars will adjust up and down for what they need and it is likely not related at all to the door jamb guidance.

It probably takes at least 6-8 psi drop to be a significant issue for handling, wear, temperature damage.

Most people will not noticed a difference in handling before 5psi change and 3-4 psi drop is not going to cause enough a problem with emergency handling.

Example math;
My car with 1200# and 33 psi 1200/33=36.364 in.² contact patch.

Drop that psi to 29 : 1200/29=41.379 in.².

5 in.² difference would likely be enough to be difference in snow not too likely to be a problem otherwise.

One case is anecdotal. Glad you achieved what you wanted but your results cannot be assumed to be universal.

Nevertheless, if tires are under inflated the contact patch becomes smaller negatively impacting handling, traction on wet or slippery roads and braking distance. I see no reason to accept any increased risk when it comes to being as safe as possible when driving especially when there are other options to address the problem that don't increase the risk.

While I agree that I'd personally prefer finding a solution other than lowering tire inflation, I always thought contact patch got bigger, not smaller with under inflation. As Andrew's math illustrates. Though sidewall might roll slightly more in HARD cornering I don't think 29psi is going to cause any major problems. I prefer getting max mpg so run slightly over inflated.

Generally, contact patch gets larger as inflation increases. But there is a point of diminishing returns. The converse is true that under inflation typically reduces the contact patch. Usually, within reason, the risks associated with over inflation are less so seeking better mileage by adding a few pounds of air can help MPG and is reasonable. I've tried some over inflation for long trips to see if I can increase mileage. Increases were not significant in my case.

In this case we are discussing contact patch in an emergency situation of braking or cornering. When turning the tires rollover to some extent. The more abrupt the turning the greater the load increase so in an emergency situation of cornering the contact patch reduces accordingly. If under inflated the contact patch will move toward the loaded side of the tire reducing the load on the unloaded side and the contact patch. The construction of the tires vary, so how much inflation it takes to reduce or increase contact patch also varies as does the point the tire breaks loose and how much it reduces stopping distance. Depending on the amount of under or over inflation determines whether adhesion is lost in an emergency situation or just regular driving.

The effectiveness of a given tire is subject to several other important variables. When discussing inflation it is important to differentiate between cold tire inflation and the normal operating temperature. Under inflation generates more heat so the change from cold tire to normal temp is greater. Outside temp is also a factor in how much the inflation changes. Most tire manufactures list 35 pounds as maximum inflation and they warn about the risk of under inflation right on the tire. Generally, too much under inflation is more than 10% of the max or 3.5 pounds.

Comparisons here should be apples to apples. As an example comparing what works for an E46, a 6 to an 8 cylinder X5, or one with sport suspension and one without don't necessarily apply. There is no one size fits all. It should be an apples to apples comparisons.

All I'm saying is there are tradeoffs, including safety, that should be carefully considered when making a decision to be outside of the manufacturers stated inflation parameters.

You can't know what will happen to contact patch or roll : Cg body roll without actual testing.

In general the only way to know what the proper inflation for your exact tire/car is will be experimentation and watching your tire wear.

I know that with Dunlop grandtrek M3 RF tires I will get edge wear = under inflated at 32. I now use 33-34 to counteract that and I haven't driven enough miles to see any wear at all yet much less more wear center or edge.

When Ford had a huge problem with explorers rolling over, the "fix" was to change OEM tires and add a couple psi to the door jamb recommendation.

That said, there is a lot of safety margin; plenty to account for 3psi drop over base recommended.

I personally would look for a better set of taller ratio non RFT tires then if that's not enough knock down the psi a little bit.

Of course one won't know what will happen based on inflation changes without some testing. That's the point. You won't know until you drive beyond the limitations of the suspension, ones talent and the tires. Yes, there is some tell from reading tire wear but that doesn't tell you what will happen when pushed to the max. I always have inside wear on my tires. They are properly inflated. The wear is from the max highway handling alignment specs I use and how hard I drive.

My bet is the OEM tire Ford changed to had higher inflation specs established by the MFG. It might have been wider or lower profile also. The Ford problem is an example of even a MFG. with extensive testing capacity that doesn't get it right. I agree there is some wiggle room for inflation. Lawyers would insist they is.

I'm sure you were using the M3 as an example of what happens with your M3 rather than comparing it to what happens on an X5 as comparisons of what happens with other vehicles is directional at best for an X5.

What happens to the overall handling of a vehicle depends on the capability of the driver, the tire and the road at that particular time. One can't set the parameters for an emergency situation.

I agree that if one chooses to improve ride with tires, the best route is a taller sidewall, a tire known for soft and quiet ride, that is not a run flat and inflated within the specs. I don't know how much is a little bit to you. Base on track experience, not with an X5, 1 pound makes a big difference to handling. I have no idea what the number is for an X5.

I notice a difference in road handing on my X5 when I lose 2-3# from ambient


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One way to get a sense of how sensitive a vehicle is to air pressure reduction is to drop air pressure in one front tire by 3psi and take a drive. The vehicle will pull toward the soft tire when one lets go of the steering wheel.

That's very true.