Tire Pressure--Who's Right?

[Brook]

I took my 2009 Mazda into the dealer for a 15,000 mile checkup which included an oil change and tire rotation (I've actually changed the oil every 5,000 miles and rotated the tires at 7,500, but this was the first visit to the dealership since I had a coupon).

The tire tag on the door post says the tires should be inflated to 29 psi which I have followed religiously. The Mazda dealership inflated them to 35 psi. Which is correct or at least better?

Speaking of disharmony between the owner's manual and the dealership, the owner's manual says to change the oil every 7,500 miles. The dealership wants it done very 3,000 miles (likely for greater income to the dealership, I suppose). I do it every 5k.

Further, the dealership did not replace one of the valve stem caps. Nice.

Any thoughts?

[Murr1525]

Re: Tire Pressure--Who's Right?

What does the max on your tires say?

Did they say they use 35psi, or is that what your gauge read when you checked it?

[FX4619]

Interesting topic for a detailing website I would follow the manufacturer and not the dealer.

[Malo83]

Re: Tire Pressure--Who's Right?

29 seems a tad low, i would run at least 32-33 psi, increasing the air pressure helps improve your gas mileage and gives the tires even wear across the tread

[juliom2]

Re: Tire Pressure--Who's Right?

On low profile tires people run 40 psi....

[GhOsT1321]

Re: Tire Pressure--Who's Right?

Your owners manual is the thing to trust.
Many dealerships go a bit above in case the customer does not take care of their tires and maybe only checks them once or twice a year, instead of at least once a month.
Tire pressure all depends on the car, our 2001 Chevy Malibu calls for 26 and 29 PSI. 26 front, 29 rear I believe.
Going by what the tire says is the wrong way to do it, that is the max PSI that is is rated for.
Also, if your tires are hot, you should overfill them by about 2 pounds or so as when they cool, the pressure goes down. The 29 PSI is for cold tires (driven less than a mile).

You can tell if you keep your tires properly inflated by how they wear. Overfilled tires wear out in the middle first as the high air pressure forces the tire into a doughnut shape. While, under filled tires will wear on the left and right side more than in the middle.
Usually up to +5 PSI is safe for normal driving. Going over a couple pounds makes the tires lose a bit of flexibility, causing them to wear slightly better, but have a rougher ride. The manufacture will recommend that which is a combination of smooth ride, and tire wear.
Having tires under inflated decreases gas mileage.

[Brook]

Re: Tire Pressure--Who's Right?

What does the max on your tires say?

Did they say they use 35psi, or is that what your gauge read when you checked it?

The 35 psi was digital tire guage reading on cold tires. I need to check what is actually listed on the tires for recommended psi. I've just gone by the tire label on the door post.

[Michael Stoops]

Re: Tire Pressure--Who's Right?

If you still have stock wheels and tires then you should follow the inflation specs on the door tag, and those should coincide with what's in the owner's manual. Some cars come standard with one size tire and wheel but have the option for a larger size as well. It is very possible that, for example, a "standard" 16" wheel and tire combo has a different inflation recommendation than the same car with the "optional" 17" wheel and tire combo, etc. But that would be reflected in the owner's manual, and the door tags would show the correct specs for the tire and wheel combo fitted at the factory.

As for the oil change interval, the old standby of "every 3000 miles" has been pretty well put to bed by most oil manufacturers and car companies as well. Many cars these days even tell you when to change the oil based not only on mileage but how hard the engine has been used, etc. These service intervals vary by quite a bit from manufacturer to manufacturer. Mercedes Benz vehicles will tell you on the instrument cluster how many miles you have left before a recommended oil change, and will then count up how many miles have gone by since you missed an oil change. Some Chevy's have a percentage countdown in the dash display (ie 87% oil life) and when it gets down to a very low number it's time to change the oil. The actual mileage isn't really written in stone.

Over all though, going with the manufacturer's recommendations is generally the smart thing to do. But keep in mind the service intervals tend to be "minimum" intervals, so if you want to change oil more often, there's no harm in doing so, but the benefits may be quite minimal. On a turbo charged engine where the turbo is oil cooled, you may want to change oil more often because that is an extreme level of heat that oil in a naturally aspirated engine will never see.

[Brook]

Re: Tire Pressure--Who's Right?

Thanks Michael. That's what I love about this forum--the quick, thorough, and well thought out responses that you and others give.

[smack]

Re: Tire Pressure--Who's Right?

You can increase steering sensitvity and road feel by going higher. Also the more weight you put in the car the more air you need to compensate. I run 40/40 in my car and 35/35 in my wifes car/commuter

[BillyJack]

Re: Tire Pressure--Who's Right?

I've been the car business for over 30 years. My thoughts on tire pressure are based on experience, as well as tire and vehicle mfr's info. On customers' tires I start with the mfr. recommended pressure and adjust for weather when necessary. For example in fall I'll overinflate by 2 psi to allow for the loss of pressure due to upcoming colder temps. On my own vehicles that I'm over, under and around all the time I'll check tread depth at the outer, inner and center grooves and let the tires tell me what they want. Ideally, the correct pressure will allow the tires to wear evenly, as GhOsT 1321 says in post #6. Typically on a FWD car, I'll be 2Lbs over sticker in front and sticker pressure in the rear, but I've been as high as 5 lbs over and as low as 2 lbs under, depending on the vehicle and driver. I also rotate every 5K miles at the most, sometimes side-to-side, sometimes front-to-rear, sometimes "X" pattern. As you might expect, with a little extra OCD care, my own tires last a long, long time.

Bill

[Wilhelm]

Re: Tire Pressure--Who's Right?

Re tire pressure-- general rule is that the higher the pressure, the harder the ride, the longer the sidewall life, the better the gas mileage. With lower profile tires the ride is pretty hard anyway and the extra pressure protects the rims a bit better from pothole damage.

I don't see any problem whatsoever with the dealership putting a higher pressure than on the door sticker, especially if they note the pressure they put on your invoice. For example, I have some weird numbers on my door sticker, something like 35 front, 39 rear. One dealership would put 32 on all four corners, another 35. When I recently when up an inch in wheel size, the tire installer put something like 42 all around. It seemed arbitrary so I asked him about it and he gave some plausible explanation. I drove up to San Francisco that same day and it was the most comfortable ride I had ever had in that car (switching from runflat tires helped). So my own bottom line is if the ride is good, it handles well, the tires were inflated by some responsible facility and the pressure doesn't exceed the tire manufacturer's limits, it's all good.

Re the oil change, of course a dealership is going to recommend frequent changes when it's on your dime. I would be particularly wary of that "rule of thumb" if they are making the same recommendation to everyone, regardless of driving style. For example, short trip commutes (5 miles or less) to work every day will murder your engine-- the oil doesn't get hot enough to evaporate the condensation/moisture in the blow by. I

n my car the oil changes were on the dealership's dime (factored into the price of the car) for the first 50,000 miles. An onboard computer would recommend oil changes based on some algorithm that included factors such as fuel consumed, miles traveled and days from last change, in descending order of importance. Based on my particular driving style, that amounted to oil change intervals of about 15,000 to 17,000 miles. I was bit leery of this at first, so would do my own oil changes in between the dealership provided free oil changes.

My particular model of car now has many that have reached over 100,000 miles and more with no problems based on the 15K to even 20K mile oil change intervals. Used oil analysis done by many owners of such cars show that even at 10,000 miles (except in severe driving conditions) the oil is still showing good lubrication, viscosity is holding up, detergents are still present, etc. This is, of course, with full synthetic oils, but modern cars well monitored as they are can easily push even dino oil 7,500 without decreasing engine life.

So my suggestions are (1) go peruse a forum dedicated to entusiasists of your particular car and see what they are saying, (2) use the 7,500 mile oil change intervals without worrying about it or (3) send a couple of used oil samples to one of the major labs like Blackstone and see how it's holding up.

[BillE]

Re: Tire Pressure--Who's Right?

A 'trick' you can try. Make a chalk line across the tire tread, drive up and down the driveway a few times. Has the line evenly worn off?

Center of the line worn=Too much air.

Outer edges worn=Too little air.

This is only a starting point, but it does help.

About the 'pressure sticker', sometimes those are WAY off. Example, my truck's sticker say 80PSI on the rear. Yea, if I'm loaded. But empty, 50PSI is the ticket.

Bill

[billoftt]

Re: Tire Pressure--Who's Right?

I use what the data plate and what the owner's manual says on my car. Afterwards, I adjust accordingly to what I will be doing. For example, on my Impala, the manual says 30 psi for all tires, I run 32 in the rear and 34 in the front. This combination, and rotating my tires every oil change has resulted in my tires lasting around 25% longer than for what they are rated. I do pretty much the same in my van, only boosting the rear pressure as well if I am loading up for a trip.

As far as the oil is concerned, I found it worth the extra 25 bucks to have my oil tested by a lab. After a series of consistent lab results, my Impala can run Mobil 1 synthetic for 9000 miles. Keep in mind that lab results will vary widely based upon the vehicle, driver, location and driving style. An example of this is my wive's van, which lab results recommend oil change intervals at 6000 miles with Mobile 1 synthetic.

I will note that regardless of what laboratory results have shown, the dealerships and quick lube shops have always recommended me to change oil every 3000 miles, even running full synthetic.

Go figure.

[GoZoner]

Re: Tire Pressure--Who's Right?

My first oil change on a new 2011 Honda CR-Z was performed based on the OBC at ~11,000 miles. Numerous people warned that the initial factory oil should not be changed early as it has special additives. Note that the first change did not require a filter change! The second change looks like it will be coming up at 22k or so. It has suggested by a Honda tech that the CR-Z measures the oil viscosity; that is, an estimation algorithm (based in starts, trip length, load, etc) is not needed.