Tire Pressure..Is there a difinitive answer?

[Walnuts]

I'll use the following data for my discussion and questions:

1995 Q45GVW 5192GVW Front 2723GVW Rear 2513Curb Weight 4039

Currently my Q has Dunlop SP5000 tires that came with the car when I bought it. The tires are rated 95T with a max load of 1521 LBS at 51 PSI.

Everytime I have taken this vehicle, or any other that I have owned, in for tire rotation, inspection, etc. the techs always look at the tire pressure listed inside the door and set it at 29psi. I have had discussions in the past with tire shops and service centers and they always say that the car was designed to run at 29 psi. My argument is, and I definetely could be wrong, that it was designed to run at this pressure with the OEM brand of tires but not necessarily all tires.

Am I correct? In my case if I want to be able use the full load of 1521 lbs I should be running at close to 51psi with these tires. If I had something else that had a load of 1521lbs at 35 psi then I would need to run them at 35psi.

Using my numbers above one might assume that we could divide the GVW for the front by 2 and get 1361 lbs which would mean that I could run at 51/1521=.033 psi per lb (.033 * 1361) = 46 psi. BTW, this is what I run the front at. The only problem is that this doesn't take into account the extra "load" that gets applied to the font in heavy breaking.

So what's the word, am I close to correct or are the guys at tires plus right?

[AZhitman]

You are correct. They are idiots.

Pressure is contingent on the tire, not the car.

Ask them what they would do if you drove in a T-bucket with Mickey Thompsons on it - Would they spend all day looking for a tire pressure sticker on a car that was built in the 1920's?

The car doesn't know what tires it has on it.

BTW, those 51 psi tires have a WEAK sidewall, thereby requiring more psi to retain their integrity.

Swap out for some 35psi tires next time, and feel free to ADD to the available margin of protection by running them at 37F/40R... You'll notice a world of difference in handling.

[PalmerWMD]

It is they who are wrong not you.29 psi is at light load and not highspeed with the origoinal Michelin which likely have stronger sidewalls than your current tires.

When I was in 15 inch I have always run 235/60-15's at 37-38 psi in front and 35-36 psi in back.

pressures in 215/65-15 likely more.

Fred..

[szh]

You are correct, not the tire techs!

Load index ratings are stated for 36psi - even for 44psi MAX and 51psi MAX tires - with deratings for lower pressure. So, that is a good value to set your minimum. Going much more than that is best done by finding a balance between outer and inner tread tire wear, handling you want, etc. Drive for a while at some reasonable value (36 to 38 psi is probably good) and adjust it accordingly.

Finally, do remember that this is COLD tire pressure - i.e., without even the sun hitting the tire in the morning. So, the right time to check and set pressure is before you exit your garage. Recheck it, at least once a week minimum, with a good tire pressure gauge.

When the tire techs reduce it to 29psi (even if it is sitting for a while at their shop), they are doing you a disservice - though, do tell them what you want done! I just recently had tires changed on my wife's car. The tech did set the pressure to what I told him I wanted (good ones will listen) - a bit more than I would normally run at. Next morning, I got out my gauge and corrected it a bit before letting my wife drive out of the garage.

Z

[maxnix]

T speed rating is woefully inadequate, and probably in itself indicates a tire of insufficient construction to hold up a Q45. Think Dennis had some very unkind words for the D5000.

[VimyJ]

The T speed rating does not comply with OEM specs. You are running at least two levels under minimum rating.

[Jeff Williams]

Tape a note on the inside of your door, with the pressure you want. Then they won't have to think too hard, next time.