Fill to vehicle spec or tire spec?

[NHHEMI]

Originally Posted By: HTSS_TR
Vehicle spec is the minimum cold pressure, you don't want to go below that. It's safe to have cold pressure at 2-6 PSI above vehicle spec, or more if you go on a long trip and your car is fully loaded, but never more than max pressure on tire sidewall.


I agree.

On my truck the door jamb sticker says 30 PSI cold which is really too low. The tire has a 44 PSI max. I set my tires cold to approx 37-38 and at operating temp it might get as high as 40-41.

 

[Astro14]

The point is this: the tire manufacturer certifies a max pressure at which the tire can operate. The vhicle manufacturer makes a recommendation and balances many things, all of which apply to that specific fitment.

Start with the vehicle manufacturer recommendation, adjust thoughtfully from there based on your needs/experience. As an example, in my '96 Explorer, the recommendation was 26 F 30 R. I ran 32 F/R. The sidewall limit was 35 (IIRC...been a while...).

Just going to sidewall max PSI gets you a tire that is over-inflated for almost every circumstance, except perhaps extreme load/towing, or racing.

 

[ARCOgraphite]

Originally Posted By: crosseyedwx
My primary vehicle specs 30 psi cold, tires 44 psi cold.

My experience has been quite the opposite... I have better tire wear, no center wear experienced ... I.
IN over 40 years of vehicle maintenance, racing and as professional NIASE mechanic for 5 years - I have never experienced the results you are getting with substantially overinflated tires; On a strut suspension car, overinflation is good way to hammer the steering ball race right out of the strut top. HIGHLY NOT RECOMMENDED.

 

[ahoier]

I'd go by the door placard, UNLESS you've upgraded your rims/tires since you bought the vehicle.

As-is the recommended PSI on the door jamb/fuel door is meant for the tires SHIPPED on that vehicle, if you go changing the size of your tires (i.e.: Go larger...) you will want to increase the PSI a bit...


True story, friend of mine got some Goodyear Viva Authority's from Walmart (great tires, BTW, mounted on the rear) they mounted and installed them on the vehicle at 32 psi according to the door placard.....well, we got 1/2 way to Tampa (about Orlando area....) and we hear a bang sound as he changed lanes, he ran over one of those reflectors in the road, and my take on it, we were likely riding on an "under inflated" (read: riding on the edge) of the tire since they only put 32 psi in the rear tires, mind you, we had a lot of weight in the back (luggage, sleeping bags, etc.....). So we pull over, change out the now-flat brand new tire, and mount the spare. Decide to turn around, in fear that the other one go out too lol and we obviously only had 1 full size spare. On the way to Walmart the next day, about a block away from the store, the OTHER tire popped off the bead, you got it; as we hit a reflector in the middle of the road, changing lanes lol.



That's my story for not going by the door placard/manual

Long story short, both tires replaced free of charge, since it was not even 24 Hours since the sale (we bought the tires at like 645p the night before, and stopped in at 10a the next morning to have the "one" evaluated, then on the way there the second one popped off the bead.

 

[Mr_Joe]

I'm usually just above the door jamb listed PSI - depending. With our big truck I'm usually riding around empty so I don't go with 80 PSI, only if towing or hauling a heavy load.

We can go back to the Explorer tires problems from years back. I think much of it had to do with low psi recommendations(listed) to get a smoother ride. Once set low it was easy to fall below a safe psi.

 

[The Critic]

3 psi above the factory recommended pressure to allow for a buffer.

 

[CapriRacer]

So what's the deal with the pressure on the vehicle tire placard.

1) It has enough load carrying capacity to carry the weight of the vehicle fully loaded.

Enough time has gone by that all the vehicle manufacturers have learned one (of many) of the lessons from the Ford / Firestone situation some 1o years back - and that is that tires should have some built in reserve at the placard pressure.

2) The vehicle was extensively tested at that pressure.

Vehicle manufacturers have a ton of people and a ton of facilities and equipment - and when they do testing, they will use the placard pressure in their tires.

3) EVERY tire manufacturer recommends that the vehicle placard pressure be used.

You would think that with all the different tire manufacturers out there, there would be one who endorses something different - but no, there isn't. NHTSA also endorses the placard. What pressure does NHTSA and the tire manufacturers use when they are doing vehicle testing? Placard!

4) The pressure on the sidewall of the tire is there because it is required by law. It is NOT a recommendation. If it says it is a maximum, it means that it is the maximum usable pressure - not that you should use it.

5) The pressure written on the sidewall comes from "The Notes on page 1-34".

http://www.barrystiretech.com/loadtables.html

About 1/3 of the way down the page I discuss this. Notice how arbitrary it is!

*****************************************

We know that vehicle manufacturers test at the placard pressure - and that makes sense that they do - but what about tire manufacturers? They aren't tied to a vehicle when they do tire tests in the lab.

Well, tire manufactuers use the rated load and rated pressure from the load tables - or something that is derived from the load table. For standard load passenger car tires, the pressure on the load table is 35 psi.

There is one exception to the pressure thing! Speed rating tests are run at 35 psi for T and lower speed ratings, H is run at 44 psi, and V and higher are run at 51 psi.

*********************************************
OK, so what about tire properties at elevated pressures?

Improved Rolling resistance (fuel economy)
Improved speed capability
Improved wear rate
Improved steering crispness

Reduced ride comfort
Reduced impact resistance
Reduced Footprint - and that has implications for 2 other things:
Reduced Evenness of Wear
Reduced Traction

I'll be honest here: There is hardly any data at all on these last 2 subjects, but I've collected enough anecdotes to conclude they are true.


Bottomline? If the person asking the question has very little expertise - and doesn't want to listen to a long winded explanation, I'll tell them to use the placard. It is ALWAYS a safe answer.

If they do have expertise and are willing to talk awhile, I can discuss all the data that is available and where it would be nice to have more data.

And to those who point to the sidewall pressure and think it has meaning? Well, I've told you where it comes from and how it is arbiratary and how tire manufacturers do not test using that pressure.......

 

[rpn453]

Originally Posted By: ahoier
As-is the recommended PSI on the door jamb/fuel door is meant for the tires SHIPPED on that vehicle, if you go changing the size of your tires (i.e.: Go larger...) you will want to increase the PSI a bit...


Is this correct? I thought that a narrower tire needed more pressure. My buddy's Audi S4 recommends 36/33 for the 245/40R18 size and only 33/30 for the 255/35R19 size. Is that an unusual situation?

 

[CapriRacer]

Originally Posted By: rpn453
....Is this correct? I thought that a narrower tire needed more pressure. .......


You are correct. Larger tires would require LESS pressure and smaller tires would require more (going with the "matching load capacity" theory)

 

[Quattro Pete]

But interestingly enough in rpn453's example, the load capacity of 255/35/19 is actually smaller than 245/40/18, unless the 255/35/19 is XL-rated.

 

[Trav]

What scares me most about over inflated tires is increased braking distance.

I like the chalk mark method mentioned in this article, i tried it on my car after the tires had warmed up after a couple of miles and the mark went away uniformly at OEM inflation.
My conclusion is the OEM spec is probably the best for this car.

http://www.nitrogentiremachine.com/proper_tire_inflation.htm

 

[Astro14]

Tires don't pop off the bead unless they weren't mounted correctly. That's not underinflation...that's bad mounting with a poor seating of the bead itself...and they DO owe you two new tires...for failing to properly mount them...

Ask Capri Racer for his take on that one!

 

[Silver]

I typically run 40 in 44 psi max tires on heavy vehicles.

I'd say run as high as you can without starting to notice harshness in the bumps, and watch for center wear and adjust if needed.

I find that many heavier vehicles are not done justice by the mfg's recommended psi. Engineer this, testing that - sometimes it just seems too low and I adjust accordingly.

More times than not, I've seen cars on car lots underinflated - I know why they do that - so the vehicle will ride smoother on test drives. For all I know, that's why many vehicles have what seems to me as low psi recommendations - so that test drive will be 'nice and smooth'.

I'm just sayin' - you never really know where some numbers come from.

 

[subiedriver]

I run the recommended 32 psi front/rear in my Subaru. This gives a good ride with good handling, doesn't seem to affect my mpg very much.

I tried 35 psi a few month's ago, it was really harsh, car has a firm suspension.

 

[CapriRacer]

Originally Posted By: Trav
.......I like the chalk mark method.....


I am not a fan of this method, because it assumes that if a tire is properly inflated, it will give even wear - and that's far from the truth.

Tire wear is extremely complex. An example of this is that steer tires tend to wear the shoulders and drive tires tend to wear in the center. That in and of itself says that the method is prone to false results.

Besides, I know that OE tire engineers (the ones designing tires to meet the vehicle manufacturer's specs) will compromise the wear in order to get rolling resistance - and one of the common techniques is to sacrifice the wear in the shoulders - making one conclude the tires are under-inflated.

 

[TacticalDriver]

8 years ago I was fortunate enough to be in Germany on the same driving training course with 2 German tactical driving instructors that were training, among others, the Berlin Stadt Polizei. From them I've learned that they are using and advocating 5-10 PSI over tire's max pressure rating.

Came back home and I tried this on 2 of our training cars (Crown Vics) 50 PSI (on all 4) without telling the trainees. To make the long story short, less experienced drivers driving the 50 PSI cars were outperforming on the course the more experienced, (known) better drivers.

Since them I have on all my 5 CV 50PSI, and on my private cars, an '06 Subaru Tribeca and an '06 BMW 550i as well as my wife's car an '08 Mini Cooper S.

Most of the police departments have adopted the over 10 PSI technique

 

[MNgopher]

Filling a tire beyond its rated pressure sounds like great advice. Not. I'm guessing there is a reason the manufacturer puts a max pressure rating on a tire.

 

[TacticalDriver]

Originally Posted By: MNgopher
Filling a tire beyond its rated pressure sounds like great advice. Not. I'm guessing there is a reason the manufacturer puts a max pressure rating on a tire.


Just out of curiosity...do you have any idea how many PSI one can pump in a modern radial tire before it explodes?

Did you ever performed a such test or do you know anyone that did?

 

[TacticalDriver]

Although is tempting to say... "I think that..." or "I believe that..." or "I am guessing...", I would like to come with empirical data, scientific explanations, laws of physics, extended personal experience, etc., and presented as arguments.

 

[flwxman]

The burst pressure is well over 100 psi for the average tire... as CapriRacer has stated on another site, and maybe here.

Suffice to say I know several people who have run their tires at or above max sidewall for hundreds of thousands of miles with zero *perceived* ill effect other than a harsher ride, and had as I mentioned, everything to gain, up to a point. That lower traction everyone worries about hasn't shown by them being in any accidents, however. Then again, they are most likely the safest and most attentive drivers on the road.

Take it for what it is. I will say it is not for everyone, and my priorities, namely fuel economy and tire longevity, are likely different from most as well. YMMV.