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Tire Pressure - 30psi required? WTF?

industrialfish

industrialfish

New member
So 3 months ago when I mounted my tires on the factory 17's I inflated them all to 35 PSI like I have for every other car ever.

Now that it's 30-40 degrees colder I noticed the tires looking a little sad and I doubt it's going to warm up noticeably anytime soon so I figure it's time to put more air in to offset the temperature change.

All tires read 32 PSI, so I go to put air in and notice a good size sticker in the door that says 30 PSI for all tires (60 PSI for spare)

30? Really? That seems a little low... Any thoughts on this?
 


Go by the tire not the door. The door stickers are a minimum requirement for factory tires.

If you can, get them filled with Nitrogen, it seems to hold up better in the cold than compressed air does.
 
Explicit_Spade said:
Go by the tire not the door. The door stickers are a minimum requirement for factory tires.

If you can, get them filled with Nitrogen, it seems to hold up better in the cold than compressed air does.
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Please don't go by the tire. That is the maximum pressure the tire can withstand.
 
Between the 98 that I had and the 05 I currently run, I got about 200k miles between them. I run 32-34 psi cold and get ~50k out of a set of tires (not the factory crap). That's not hitting the wear bars either. Running 30 psi on the GPs will ruin the tires fast.
 


Its very difficult to over inflate a modern radial.

Very easy to under inflate it.

Mid 30's is fine for daily driving.
 
Thanks guys, that's what I thought. I've always put 35 in every normal tire I've ever ran or mounted for others unless for a specific reason otherwise. Just wanted to make sure there wasn't a GP specific reason for the 30PSI sticker.
 
My factory recommendation sticker and tire recommend 30 PSI, but I put it at 32 PSI since my driving wears the outside tread more than the inside (at 30 PSI).
 
Nitrogen is not going in my tires if I have to release some of the pressure twice a year to reset the TPMS when switching summer/winter sets.
 


tod1d said:
Nitrogen is not going in my tires if I have to release some of the pressure twice a year to reset the TPMS when switching summer/winter sets.
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nitrogen seems to not be affected by the cold or heat as much and just air. nitrogen molecules are larger then air molecules the tires should remain in a good ball park going into winter.
 
GTP in RTP said:
What does the difference front to back accomplish ? Better handling ?
Click to expand...

the only thing i can think of to why this was said is that the rear suspension typically will have more natural negative camber (not adjustable stock) and will wear more unevenly if you don't rotate the tires like a normal person.

otherwise i have no idea why you'd run different pressures all around the car.
 
Bio248 said:
the only thing i can think of to why this was said is that the rear suspension typically will have more natural negative camber (not adjustable stock) and will wear more unevenly if you don't rotate the tires like a normal person.

otherwise i have no idea why you'd run different pressures all around the car.
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Front wheel drag racers go low in front for more grip and high in back for faster rolling. For a GP daily driver I don't think the difference between 32 and 35 would be noticeable... however bigger aftermarket wheels usually have lower profile high performance tires and require higher pressures. My car has 18" wheels and I normally go with right around 40 psi. http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=196
 
no, i understand that. however, 32 isn't "low" per the grip standards of going to the track so i was assuming they were driving it around like that. it doesnt make sense to have the front and rear at different pressures...
 
It was always recommended pressures on several different cars I have driven, so it is something I have stuck with.
 


I want to clear up a few things on this zombie thread for those searchers like me.

What is tire sidewall psi?
The psi on the tire sidewall (typically 44 psi for P-type passenger car tire) is the maximum inflation pressure the tire can withstand and carry the highest load.


What is the manufacturer psi on the door?
The psi on the vehicle tire and loading information placard is the psi recommended for the vehicle. This number comes from the manufacturer (GM) based on the weight of the vehicle. The manufacturer specs the tire size they want (cost, top speed, comfort, looks, availability, etc), calculates the maximum loaded weight of the vehicle, adds in a safety factor (SF) for overload or uneven loads, then looks up the tire in the Tire and Rim Association (TRA) load inflation tables for the correct psi. (sometimes manufacturers may increase the psi above this to get mpg or handling targets at the sacrifice of comfort.)


What about my stock tires?
So for example for my stock P225/60R16 tire it is rated for a max load of 1,477 lbs per tire at 30 psi. (Always checked when cold prior to operating the vehicle) GVWR is 4,400 lbs / 4 gives us 1,100 lbs per tire (of course it will be higher or lower depending on location and loading). We have a safety factor of 377 lbs (ignoring front/rear weight bias). The 30 psi provides enough pressure to properly support the vehicle's weight, give good tire wear, and balance comfort vs. fuel economy.


Where does 35 psi come from?
P-rated passenger car tires are generally weight rated from 26 lbs to 35 lbs (some up to 42 lbs). So many tire shops play it safe and recommend 35 psi so you do not exceed the weight carrying capacity of the tire. (Too much weight on the tire vs. psi will cause it to flex more, overheat, and premature failure)


Why do some cars have different front and rear PSI recommendations?
Primarily the weight of the vehicle is different between front and rear and they want to ensure a tire is not overloaded. Additionally the manufacturer may be using tire pressure to achieve a desired tire stiffness for handling performance (oversteer/understeer).


What pressure should I run in stock tires?
30 psi provides the psi necessary to carry the weight of the vehicle when fully loaded. It provides a balance of comfort vs. fuel economy and even tire wear.

For my P225/60R16 tire it has the following weight capacity: 26 psi/1,378 lbs..... 29 psi/1,455 lbs..... 32 psi/1,521 lbs..... 35 psi/1,609 lbs.
You cannot, I repeat, you cannot judge a tire based on squish or the pudge on the rim. The squish is based on the carcass of the tire (stiffness, # plys, profile, design, etc), sidewall height, weight of the vehicle, etc). Some tires look more pudgy than others and some even look flat at the proper psi. Use a tire gauge!


What if I want to run lower tire pressure?
If you run a lower tire pressure, say 26-29 psi, you may get better traction as the tire will flatten out more. You will get more comfort. You will get decreased weight carrying capacity and may risk overloading the tire. You will also have decreased handling and decreased fuel economy as the tire is flexing more. If you run a lower tire pressure ensure you are not exceeding the weight limit vs. psi and watch for uneven tire wear.

You will need to know the weight at each corner of the vehicle when fully loaded. Take the highest weight and look up your tire size in the TRA load inflation table. Ensure the highest weight on one tire does not exceed the maximum weight carrying capacity of the psi you decide to run. If you go on a family trip and load the vehicle more than usual (or tow a trailer) ensure you increase the psi to account for the extra weight. Think drag racing low psi to flatten out tire for max traction.


What if I want to run higher tire pressure?
Many people run 35 psi, 40 psi, or even max 44 psi. You will get increased weight carrying capacity, a stiffer tire, and increased handling. The tire will be stiffer so you will lose comfort. It may increase fuel economy and possibly cause uneven tire wear. You may get less traction as the tire will assume a more rounded profile. Increase psi a little at a time to meet your desired results. Don't exceed max tire psi (44 psi) and steer around potholes! Think hypermiler to get less rolling resistance at the sacrifice of comfort and tire wear.


What about Nitrogen?
Nitrogen is a good marketing tool (aka profit) for tire dealers but not required or necessary. Nitrogen filled tires maintains a more stable psi with slower pressure loss. It still doesn't replace checking your psi every several months (especially as weather gets colder). Air is 78% nitrogen. Feel free to use nitrogen for more stable tire pressures but it is certainly not necessary.


What psi do I run for if I change tire size?
First you will need to look up the tire size you want to run in the TRA load inflation table. Look at the psi vs weight to ensure you run enough psi to handle the weight of the vehicle with a safety factor.

Lets use my example. I switched to P225/55R16 since there are not many performance based tires in our stock size. I went to a lower profile tire which has a decreased weight carrying capacity. Looking up my new tire size I see that at 30 psi the tire can carry 1,367 lbs. Too low. Since GM spec'd 1,477 lbs per tire I need to increase my tire pressure to 35 psi (1,477 lbs) to ensure I can carry the proper load and maximum GVWR.
 
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Ugh, nitrogen. If you pay $0.01 for nitrogen in your tires, that's WAY too much. I wish nitrogen would just go away.

Some additional rules of thumb:
1) Tires increase/decrease about 1PSI per 10 degrees of ambient temperature. (Nitrogen or not)
2) For a lot of highway driving, I will inflate tires to about 4PSI over the sticker pressure. Much more than that, and people will start to notice a harsher ride. Doing so will yield about 0.2-0.3 MPG improvement. I run mine at 5PSI over sticker.
3) For low-speed local driving (i.e. mom driving her kids to school) I set to the factory recommended pressure because too many other factors influence MPG for that type of driving, and they don't need the harshness.
4) The higher the tire pressure, the less likely you are to damage a sidewall if you hit a pothole. This is another reason you need to check your PSI in the winter - colder weather (i.e. lower PSI) PLUS more potholes...
5) When you get rid of your old tires, look at how they are worn, If the centers of the tread are worn then your tire pressure was too high. If your edges were worn, your pressure was too low.
6) Higher weight = higher PSI. For example, extra weight in the front (engine) means you need a higher PSI. If you tow or put a lot of luggage in the trunk (or have heavy passengers in the back seat) then bump up the PSI on the rear tires a bit.
7) Don't get too excited about changing your PSI at the drag strip, unless you run a drag tire. The lower PSI on drag tires allows the sidewalls to flex, but doesn't have nearly the same benefit on a street tire.
8) PSI tends to climb about 3 PSI when highway driving due to temperature increase (2-4 PSI depending on a bunch of factors).
9) I rotate my tires about 2-3 times over the life of the tires, and keep an eye on treadwear. You don't need to rotate/balance every oil change.
10) Put your best tires in the rear, especially in winter. That gives you stability in a panic stop- if your car stays straight, at least you can still steer... if your rear end swings around because the tires suck, then your steering wheel becomes pretty useless too. Oh yeah, change your rear shocks/struts at about 75k miles too, they are more important than the fronts.

-BC
 
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