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Tire pressure


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Bertimus

I know I have seen this topic discussed before, but I could not find the thread after a lengthy search. I have Cooper Cobra radials on my 88. They are the factory correct size of p215/65r15 95T. I was wondering what psi is best, especially considering the front end is much heavier? My preference is shock absorption over tire longevity, since I don't drive her much. Thank you!

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DAVES89

Look on the door of your car or the owners manual

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Bertimus
6 hours ago, DAVES89 said:

Look on the door of your car or the owners manual

Oh definitely that was the first thing I did, but I was told that tire technology has come a long way since 1988, and that the recommendation is now somewhat obsolete. I once found a thread where you guys were all listing your own preferred psi, I noticed a lot of you guys even had different psi for the front and back end... I couldn't find that thread so I was hoping that some of you guys would post your psi again 🙏🏽

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Ronnie

I usually run 35 in the front and 30 in the rear. Most of my driving is on curvy mountain roads and drive pretty fast around the curves sometimes. The higher pressure in the front seems to make the car handle a little better. Lower pressures might give a better ride but I think the FWD cars tend to wear the outside edges more on curvy roads if you allow the pressure gets too low.

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DAVES89

I run 34 to 36 in both front and rear...

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2seater

Yes, I responded on Facebook too. Weight distribution is approx. 65%Front, 35%Rear so I use more in the front, pretty much the same as Ronnie. That said, Dave puts lots of miles on and what works for him long term carries some weight in my opinion. I run directional tires so conventional rotation patterns to equalize wear aren’t possible.

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Padgett

" tire technology has come a long way since 1988, and that the recommendation is now somewhat obsolete." Not true, is still valid for stock tires, that said I have been running 225x60x16s on factory 16x7 wheels (like crosslaces) for  several decades. Fiero had bigger wheels and tires & never thought 215s looked right.

 

ps biggest change has been the addition of a nylon cap/ply under the tread, keeps the tread from coming apart.

plies.jpg

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Philbo

As someone who works for a tire company, I would recommend sticking with the pressures on the door. It's more complicated than this, but generally, tires are designed assuming you are following those recommendations.  Mostly it is about having the proper footprint for even wear over the life of the tire. Running the pressure higher will generally cause the tire to wear faster in the center, and lower pressure will tend to cause shoulder wear. Of course this also depends on having proper front end alignment as well as rotating your tires regularly.  This is kind of over simplified, but you shouldn't go wrong by running what's on the door.

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2seater
4 hours ago, Philbo said:

As someone who works for a tire company, I would recommend sticking with the pressures on the door. It's more complicated than this, but generally, tires are designed assuming you are following those recommendations.  Mostly it is about having the proper footprint for even wear over the life of the tire. Running the pressure higher will generally cause the tire to wear faster in the center, and lower pressure will tend to cause shoulder wear. Of course this also depends on having proper front end alignment as well as rotating your tires regularly.  This is kind of over simplified, but you shouldn't go wrong by running what's on the door.

Agree in general if running tires and wheels with matching specifications to the OEM fitment. Many of us have changed wheel diameter, width and offset, plus tire diameter, aspect ratio and width, as well as type of tire, so I do believe a bit of custom tuning may be necessary. One item I have noticed over the years has been some lack of concern for the speed and load rating of the OEM tires. I don't remember the load rating offhand but the factory H speed rating is 130mph, which does effect the tire construction. I have seen occasional references to tires that are a good deal and maybe a long tread life but a lower "S" or "T" speed rating. Probably fine for a cruiser but would fall short of the factory design parameters.

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Philbo
20 hours ago, 2seater said:

Agree in general if running tires and wheels with matching specifications to the OEM fitment. Many of us have changed wheel diameter, width and offset, plus tire diameter, aspect ratio and width, as well as type of tire, so I do believe a bit of custom tuning may be necessary. One item I have noticed over the years has been some lack of concern for the speed and load rating of the OEM tires. I don't remember the load rating offhand but the factory H speed rating is 130mph, which does effect the tire construction. I have seen occasional references to tires that are a good deal and maybe a long tread life but a lower "S" or "T" speed rating. Probably fine for a cruiser but would fall short of the factory design parameters.

Yes you are right, if you are running a different size/setup all those assumptions kind of go out the window and a different pressure may be needed however unless you are using something completely off the wall, the factory recommended pressures aren't a bad place to start.

 

As for speed rating. The speed rating isn't exactly the top speed of the tire but the highest sustained speed that tire is tested for at load. So an S speed rated tire is 112mph. So unless you plan to drive long distances on the highway above that speed, I wouldn't really worry about it.

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Padgett

Am more concerned about load ratings. Also GM cars were designed for P-metric tires. While speed rating gives some information, LT tires should not be driven over 65 mph (some allow 70 with an additional 5 psi), the construction is even more important. A "C" rated tire is 50 psi, D is 65 psi and E is boy howdy. Also a P-metric can be Light Load, Standard Load, and Extra Load all of which make a difference that was not a concern in 1988.

 

BTW I also look at the Load Index (generally in the 90-110 range for a passenger tire) and the UTQG which indicates the tire wear: 200 for a track day tire and >400 for a street cruiser.

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