Fractal

Hello. Longtime listener, first-time caller. I love this place.

I would like to hear opinions regarding my Dealership intentionally over-inflating my tires.

So, a day after I took my 2009 LS460 AWD in for the 5K maintenance I checked the tires’ air pressure. I suspected they were high because the TPMS said they were all 39 - 40 lbs hot. Two quality air gauges said they were all 36 pounds cold. The Dealership had just exchanged my Winter tires for my All Season 18” 235/50 Turanzas. Recommended pressure is 33lbs.

So, I called the Service Rep and he said Lexus told them to over-inflate the tires to reduce / eliminate the number of Low Pressure Warning Light cases. He said they used to get a lot of them before they added the 3 extra pounds. He said they put 36 pounds in all LS460s. I asked him about a TSIB and he said there wasn’t one because there were no parts involved. He said Lexus told them to take this action during a weekly telecon.

So, would Lexus instruct their dealerships to do this? What do you think?

Mike_in_DTW

I'm not sure what the logic is, but mine were all over inflated when I got my car. Maybe they think we're all too dumb to keep our tires properly inflated after the Firestone / Explorer problems of a few years ago. Coming out of an 06 LS430 I was a little surprised by the ride of my new LS460L. Reducing the tire inflation to the proper levels made an improvement, but it's still doesn't ride like the LS430 it replaced. I love the new car, but I miss the soft and quiet ride of the previous car. Just my $.02.

jmcraney

You are going to get a lot of input on this, but here is what I think you need to know.

Tires loose pressure by permeation (the process of air molecules transiting through the tire, sort of like what happens overnight to rubber balloons, but not so much with mylar balloons - filled with helium). Permeation loss of up to 1 PSI per month is considered normal although .5 PSI is more typical. The pressure in tires vary as a function of temperature, up to 1 PSI per 10 degrees Fahrenheit - up for warmer and down for cooler - but typically .5 PSI per 10 degrees.

So, as an example, if we adjust the inflation in our tires for 33 PSI on a morning when the temperature is 80 degrees and then check them six months later on a morning when the temperature is 20 degrees we would probably find that the pressure is 27 PSI - a 3 PSI permeation loss (.5 PSI per month X 6 months) and a 3 PSI decrease due to temperature (.5 PSI down per 10 degrees X 60 degrees). If the TPMS benchmark for all tires was 33 PSI then the alarm threshold is about 28 PSI (15% below the benchmark) then you have an alarm situation.

Conversely, if we adjust our tires to 33 PSI on a morning when the temperature is 20 degrees and check them six months later on a morning when the temperature is 80 degrees we would probably find that they are at 33 PSI.

So, you can see that when there is a sudden major decrease in temperature the dealerships have lots of cars coming in with suspected TPMS problems.

You should be able to set your tires for 33 PSI at the coolest time of day and add air once every three months and not have any problems. Seasonal changes present challenges to this in that when the temperature increases it sort of offsets the permeation loss but a sudden temperature drop along with prolonged permeation loss presents an alarm condition.

All tires behave this way. If you use nitrogen instead of air the permeation and temperature coefficient is marginally better but not worth the trouble or the cost. You have to add more gas - nitrogen or air - periodically.

Because the spare tire is somewhat difficult to get to you should inflate it to 40 PSI once a year. You must avoid benchmarking your spare at 40 PSI. If you decide you need to benchmark your tires be sure that the spare is at 33 PSI when you do that.

As for the accuracy of the TPMS sensors: I have every reason to believe that the sensors are accurate and temperature compensated. I would rely on the numbers displayed on the dash unless you have proven that they are not accurate. You must keep in mind that the display resolution is 1 PSI, so tires displaying 32 and 33 may be very nearly the same pressure or as much as 2 PSI apart and you have that same problem with the gauge you are comparing with. I would use a single, high resolution gauge to set the tires.

daryll40

I've kept my tires at 33PSI religiously since I got it in October 2007. Never once have I had a "low tire" light come on.

jmcraney

As Daryll40 has said - I sort of misled you in my example - the correct cold inflation pressure for the LS460s tires is 33 PSI.

Thank you Daryll40!

link13

I don't think 3 lbs extra is a big deal. I tend to run the tires on my GS at about 35 lbs. I want them to crown a little bit to reduce wear associated with camber issues.

alsilverbullet

I also keep my 22's at 35psi cold. When hot they go up to 38psi which makes for a comfortable ride..

Fractal

I tried different air pressures while driving a 2003 Avalon for 120,000 miles. I liked to keep the pressure at 33 lbs, one pound over recommended pressure. When I discovered the Lexus Service Department had over-inflated my beloved LS’s tires by 3 pounds I liberated 2 of them and am running one pound over @ 34.

The negative effects of under-inflation are well known but you can’t find much info about over-inflation. I don’t know much. Some people claim to run at max inflation. I would not do that. After some time I will increase the pressure to 35 and see how I like that. I might even try 36 again, who knows? But that will be my absolute max pressure unless I learn something knew, which experience has taught me is unlikely.


Now, back on topic. I believe it is wrong for Lexus to intentionally exceed the recommended tire pressure simply because too many cars have warning light problems. Some engineers at Lexus determined that 33 lbs was the best (recommended) pressure for my tires. Sadly, they did not know there would be too many Low Pressure Warning Light problems for the Service department’s taste.

Remember, they do this without consulting the customer. That is wrong. If the Low Pressure light is a problem then fix the problem - seems obvious. Is 3 pounds over-inflated a safety issue? Probably not. A tire-wear issue? Dunno. But, what if you like to follow all recommendations? I’ll bet some folks clean their coffee makers as per Mr. Coffee’s recommendations, which is their right. Bless their hearts.

In closing, I may inflate my tires as I please but YOU had better put 33 cold pounds in them. Of course, if you want to convince me that 36 is a better number than 33 that is your right.

BTY, I’ve never had a Low Pressure warning light with my Winter tires at 33 or with the Turanzas which had 33 when I took delivery of the car.

I gotta go check my tires.

Gojirra99

I can feel the difference. Last time I bring my LS460L in for service, they inflate them from 32 to 35, I noticed the stiffer ride immediately after I picked it up.

Mike_TX

I personally doubt Lexus corporate told the dealers to do this. It is far more likely the dealer got tired of lame brains coming in with warning lights due to low temperature contraction.

And I agree with Gojirra - I can feel a couple extra pounds of air in my tires. I'm so **** about tire pressure that I keep the pressure display up on my gauge cluster all the time, and if I see an unusual fluctuation, I'm checking the tires and adjusting the pressures.

Unfortunately, most people don't do that, though, so your dealer is just making it easy on themselves by over-inflating.

A side note: When you pick up a new car, odds are the tires will be inflated to 40-45 psi ... the reason is that cars on the lot (and during transit) are over-inflated to avoid flat-spotting of the tires. The make-ready people often (usually) fail to re-set them to the correct pressure.

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I8ABMR

The extra pressure is going to help avoid warning lights and I have to admit the 36-39 psi is not going to hurt you.

PatsSoxfan

"All tires behave this way. If you use nitrogen instead of air the permeation and temperature coefficient is marginally better but not worth the trouble or the cost. You have to add more gas - nitrogen or air - periodically."

Jeff,

I have found that Nitrogen is a real ripoff and is actually considerably worse (more temperature sensitive) than plain air. I have to make about 3 trips to the dealer for my all season tires in the spring to reduce the pressure and 3 trips in the fall to increase the pressure with this nitrogen crap. My snow tires do not have anywhere near as much problem since they are filled with plain air.

I am really ticked at Lexus for recommending nitrogen for a cost of $50. The claim was that I would never have to worry about tire pressure with temperatures changes again. What a bunch of balogna that was!

BTW, you folks living in the South have no idea about sharp temperatures changes. In the spring, here in IA we can go from 70 degrees one day to highs in the low 30s the next day. In the latter part of the fall it can be even more dramatic. In late November to early December we can have temperatures in the low to mid 60s one day and zero or below the next morning. Or how about this example in the middle of winter? 50 degrees one day and 15 degrees below zero next. Thus, it is MUCH more challenging keeping a level tire pressure here than in the South or in the West Coast.

Mike_TX

LOL. We don't go to minus 15 in Texas, but 50-degree temperature swings in one day are nothing for us. Happens all the time.

The problem with nitrogen in car tires isn't the nitrogen - it's the method used to fill the tires. The only way to get a good concentration of nitrogen in a tire is to purge the plain air and replace it with nitrogen. But all most places do is pull the valve stems to let the tire go to ambient pressure (equal to the normal air pressure that's around us all the time). Then they stick a hose on and fill the tire back up with nitrogen.

Problem is that there's still a LOT of air remaining in the tire. And at average atmospheric pressure of 14.7 psi, you can see that when the tire is "aired up" to, say, 33 psi, only about 2/3 of the pressure in the tire is nitrogen. And even at that, the nitrogen they use isn't "pure", so you end up with maybe half and half air and nitrogen at best.

Then, if you're out in the boonies and need to air up your tires, boom! - there goes more of your nitrogen concentration.

I think you're better off spending your 50 bucks on cheeseburgers than wasting it on nitrogen for your tires. But that's just my opinion ... after having used it for two years in an Infiniti.

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jmcraney

The sea level composition of air is about 78% nitrogen, maybe not so high here in Texas where Mike and I live. You can get the nitrogen concentration in your tires up pretty high with a little effort. The more important thing is to make sure your tire air is dried properly.

Patssoxfan, as for the difference in your snow tires, they probably have a smaller permeation coefficient. 100% nitrogen behaves much like air (78% nitrogen).

http://www.physlink.com/Reference/AirComposition.cfm



Quote from the link above:

Air Composition

The sea-level composition of air (in percent by volume at the temperature of 15°C and the pressure of 101325 Pa) is given below.

Name Symbol Percent by Volume
Nitrogen N2 78.084 %

Oxygen O2 20.9476 %

Argon Ar 0.934 %

Carbon Dioxide CO2 0.0314 %

Neon Ne 0.001818 %

Methane CH4 0.0002 %

Helium He 0.000524 %

Krypton Kr 0.000114 %

Hydrogen H2 0.00005 %

Xenon Xe 0.0000087 %

Fractal

I would like to thank Mike TX for his on-topic reply. Thanks Mike TX!

I agree with Mike TX that it is unlikely Lexux Corporate instructed dealerships to over-inflate tires.