Topic: At what pressure do I run my replacement tires at - A formula

[Critterman]

Saw this question from kenny and while on a truck forum I came across the same question, but there was a formula in the answer.  I though I would share it here and let the engineers in the group have a go at it.

X*35%/Y*Z

X= Gross weight
Y= max load per tire
Y= at what pressure

Using my N/A as an example and guessing at 3350 gross weight and the stock RSA tires at 1565 at a pressure of 44 lbs we have:


3350*.35/*44=32.9
Since the factory says 29 lbs I dropped the persentage to 30%
3350*.3/1565*44=28.25.  
of course it all depends on my actual gross which I am guessing.   BUT based on that my Sumitomos to give me the same ride should be at 26.lbs  since they are rated at 1929 @ 50 lbs  at 35% I would be running 30.4 lbs.

Okay have at the formula and let me know where it is wrong.

[Sly Bob]

Made this a sticky. The logic sounds good John. At the very least, that gives someone a starting point from which to experiment.

[kennysabarese]

I have seem formulas for calculating tire pressure based on vehicle weight on the web in the past. Weight directly affects the contact patch.

 A friend of mine has a Porsche 911 and when he put on aftermarket tires, the manufacturer (whom I can't remember) told him to run them much lower than the stock door tag.

On the track we use chalk or shoepolish around the edges, and see where it wears off to make sure the entire part of the tire that is intended to touch the pavement actually is.

[Uranium-238]

On the track we use chalk or shoepolish around the edges, and see where it wears off to make sure the entire part of the tire that is intended to touch the pavement actually is.

That's pretty much what we did with my tires, though in my case no chalk was needed, pretty obvious where the wear was.

[Critterman]

I have chaulked the Sumi's, and as low as 28 they don't roll on to the sidewall.  Of course I am running a negative castor front and back

[Treeman]

Critter,

This formula works for the stocks but you had the wrong vehicle weight - its about 3,000 lbs.  So -

X*35%/Y*Z

X= Gross weight
Y= max load per tire
Z= at what pressure (correcting typo, also this is max tire pressure)

3000*.35/1565*44 = 29.5 psi

For my new tires, Y = 1764 and Z = 50 psi

3000*.35/1764*50 = 29.76 psi. 

So, same pressure since I am not worried about a quarter psi.

These are cold psi.  And yes, for autocross I while use the chalk method and consider looking at the same hot psi as on the road. Autocross is a special animal.

[Critterman]

I included me in the seat

[TomatoSoup]

Interesting, but it may well be different from tire to tire.  My Conti DWS's are the same nominal size as the OEM G2's, but they need much more pressure to be optimum.  I run them at 33-34lbs now.  I believe DBG would agree with my premise - he has the same tire.

[Treeman]

I included me in the seat

Oh, my!!! Then you underestimated!!!!   

[Treeman]

Interesting, but it may well be different from tire to tire.  My Conti DWS's are the same nominal size as the OEM G2's, but they need much more pressure to be optimum.  I run them at 33-34lbs now.  I believe DBG would agree with my premise - he has the same tire.

The tire differences are specified in the load range and the maximum pressures, which can be found for the tire at Tire Rack.  That being said, I ran the stock GS-2s at 33psi - but that was to overcome what I saw as shortcomings in the tire itself. I would not argue with anyone that they optimum contact patch for that tire was 29 psi. 

Of course, each of us seems to find something that works best for each of us. 

[Graywolf]

You should not have to chalk your tires but should be able to feel the tire rolling under on corners[ Bad ]with under inflation.  Tire pressure between 30 to 34 lbs works best for performance driving on the street[Cold tire pressure] Your tire pressure will go up 2-3 lbs just from driving around. Higher pressures will make your car feel the bumps in the road harder{Sucks]. Road racing I start @ 34lbs cold and the pressure goes up to 37lbs after one hot lap. Play with your pressures, Suggest you start @ 34lbs drive your car around, measure the pressure and drop 2lbs from the measuement that you get[i.e. if it measures 36lbs go to 34 with warmed up tires, then repeat the procedure and drop 2 more lbs etc. .Tire pressure should be adjusted to what feels good to you! Formulas work great as a starting place

[Go-N Def]

Interesting, but it may well be different from tire to tire.  My Conti DWS's are the same nominal size as the OEM G2's, but they need much more pressure to be optimum.  I run them at 33-34lbs now.  I believe DBG would agree with my premise - he has the same tire.

When I first got my DWSs, I was running 34/36psi (Approximate cold/warm) which was done by the shop that put them on.  The car felt like it was sliding around A LOT more than the stock tires.  I keep them at 29.5/31.5psi now and it hugs the road better for me.

[DickW]

I run my Conti DWS filled with nitrogen at 31. I use nitrogen because you dont get the pressure variations in winter.

[Brad Kenny]

Interesting, but it may well be different from tire to tire.  My Conti DWS's are the same nominal size as the OEM G2's, but they need much more pressure to be optimum.  I run them at 33-34lbs now.  I believe DBG would agree with my premise - he has the same tire.

I also run my Conti's at 33 pounds cold. The max I have ever seen after 500 miles driving was 39-40 pounds. Might wanna try 31-32 to see if I like the feel any better for long trips. Thanks Go-N-DEF.

[TomatoSoup]

UPDATE:  Based on posts on the BB, I see that the formula above is not correctly written - it is missing parentheses.  Corrected here for future viewers:

    Inflation PSI = (X*35%/Y)*Z     (<- parentheses added)

- OR (AKA) -

    Inflation PSI = Z*X*35%/Y

    Where:
        X= Gross vehicle weight (lbs)
        Y= max load per tire on sidewall (lbs)
        Z= max tire pressure on sidewall (PSI)

For our cars (stock tires) where X=3000; Y=1565; Z=44, this gives:

    44*3000*.35/1565 = 29.5 psi

[Sol Asylum]

Are you saying Critter mistyped that too?

[ihawk95]

Interesting, but it may well be different from tire to tire.  My Conti DWS's are the same nominal size as the OEM G2's, but they need much more pressure to be optimum.  I run them at 33-34lbs now.  I believe DBG would agree with my premise - he has the same tire.

Interesting!  I was thinking I had this tire, but then realized I have the DW and not the DWS. I will have to see if the max load/pressure are different vs the DWS.
Anyway, I ran the tires with more pressure at the track and we did mark my tire just to see what it really was doing and it looked good.  As the day went on, the tires warmed up a lot and we let air out.  I can't remember where I ended up.  I want to say it was around 34 or 35 ish.  Damn I'd wish I'd written that down!  Now I'll have to play with it again next time.