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Tire Pressure

This is driving me nuts. I own a 2004 Volkswagen Passat GLX, 6 cylinder. The door placard says to inflate the front ties to 36 PSI and the rear tires to 44 PSI. The dealer says to inflate to 33 PSI all around. I just bought new Michelin Primacy tires and the tire dealer says his book says to inflate to 28 PSI, but inflated it to 32 PSI. Does anyone know the correct tire pressure for this car.

Thank you

The door placard is always the number to follow, as long as your tires’ maximum pressure (on the sidewall) isn’t exceeded (if the placard recommends pressure higher than the tire allows, your tire dealer sold you the wrong tires - happened to me). I am surprised at the high pressures, especially in the rear, but that’s what the factory wants. Ask your dealers (tire and car) to document their recommendations in writing. Where do they get that information?

some placards will give varying pressure depending on the tire size and loading conditions, so pay attention to that closely…use only tires that are recommended for this vehicle, too many people buy the wrong type or size sometimes pressured by tire dealers for what they make the most on or have in stock.

I suspect the door placard is not correct. 32-34psi all around, you can’t be too far off. EXACT tire pressure is not critical…A couple of PSI up or down will not make any difference…

I think you’re right, Caddyman. The Firestone tire pressure web site has much lower values, too. The dealer should be able to get the right #s.

According to Tire Guides, there are 4 versions of Passats - and 7 possibilities for tires size and inflation pressure. Some are as low as 28 psi and some are as high 32 psi.

But the one thing Tire Guides doesn’t do very well is deal with placards that have different pressure for different load/speed conditions.

As RedTag said above, pay attention to the tire size and the conditions indicated on the placard. Placards are rarely wrong, and if they are, the vehicle is recalled and re-placarded (Is that a word?)

If the vehicle has a different tire size than is listed on the placard, then the pressure has to be recalculated. It’s not difficult, but requires the use of tire load tables, which aren’t allowed to be published on the internet. You have to buy the book at $80.00 each - and I have one and am willing to do the work.

This has been a good thread. Lots of correct information.

The two authorities are the tyre manufacturer and the car manufacturer.

The tyre manufacturer knows the MAXIUM pressure the tyre can be safely used at.  DO NOT exceed that value.  If your car manufacturer calls for more, then you have tyres are are not safe on your car. 

 The car manufacturer has done extensive test track work to determine the safest tyre pressure for their car.  However that only applies to the size or sizes of tyres that they recommend.  If you have a different size tyre, it involves some guess work.

  One or two psi difference is not serious.   But don't vary far from the car manufacturer's recommendation.  Some people will tell you to go higher or lower and don't worry or that they personally have determined it is safer.  Unless they have personal access to the advanced testing facilities of at car manufacturer then they are just guessing based on incomplete information.  It is those really extreme conditions that may occur in an accident situation that are really important.  Stick with the car manufacturer's recommendations.

The first two replies are spot-on. Additionally, it is important to maintain the front-to-rear pressure difference to maintain the handling characteristics (basically, oversteer or understeer) VW designed the car to have.

The best tire pressure is the pressure that keeps the optimal amount of tire surface on the road. This is based on tire size and weight of the vehicle. If the size of the tire changes or the weight changes the optimal pressure will be different. I’m also very surprised at the numbers on the door jam. That seems way too high for a vehicle of this weight…

[i] The best tire pressure is the pressure that keeps the optimal amount of tire surface on the road. [/i] 

NO. While that might provide the “most” friction under some conditions, that is not always best. Under emergency conditions, which is where your life Is on the line, you want the car to handle best and that does not always include the most friction Balance front to back can be very important, too much friction can be bad.

Yea, let me repeat that "too much friction can be bad."  If you want to know why Google "Ford Explorer Firestone"

I’m surprised that a FWD car would recommend a higher pressure for the rear tires than the front. Are you sure of those numbers on the placard?

The placard gives tire pressures under different loading conditions for the vehicle suspension, wt, and other critical parameters. This differential may sound high, but under certain conditions is EXACTATLY what the mfgr. wants. And by the way, a couple of pounds in tire make a very big difference…People lowered the Firestones on their EXplorers to get a smoother ride, but caused severe overheaating and a lot of tire failures that killed some people, so pressure is very important.

“And by the way, a couple of pounds in tire make a very big difference…”

NO, it doesn’t. 2psi makes literally NO difference. Tires change 6 or 7 psi all by themselves depending on temperature. Call it self regulating speed compensation.

Pressure is temperature related so the local and local conditions and road type made big differences on how these tire perform…26# at 95F will be a different pressure at 46F yet the hightemps caused cord flex, overheat and failure. Most accident were in unusually hot climates and certain types or use and road surface. And 2-3pound did make a surprising difference with these vehicle and tire combo.

Yea, I don’t believe it is that unusual. With a full load the weight distribution is close to even. In most cars they want the car to have under-steer. It is generally safer for most drivers in emergency conditions.

Yes pressure changes based on temperature. But when you start too high or start too low and then you add in the temperature caused difference, you have a problem. The recommended temperatures take that into account.