Tire pressure question about 19 inch wheels

[Reg]

Ok, I have only had the car for 3 years and 30k miles, so I am to be forgiven to finally trying to figure out tire pressure

I have a 4S with 19 inch wheels. The display and the online documentation is saying the rear tires shoud be 32 psi, front are at 38 Whenever they came back from service they were at around 37 or 38, which is how I have always driven the car.

I don't understand why the rear tires would have such low pressure compared to the 20s.

That seems really low to me, and afaik, would mean that lowering them to that level would improve traction at the cost of efficiency.

What am I missing? Why woud the rear tires be low?

thanks

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[Archimedes]

By law in California, a dealer/servicer is required to return your car with the pressures set to max GVWR. It’s a legacy of the Firestone/Explorer debacle.

I would set based on what the manual says, though I always err slightly on the high side. Just remember to always set the pressure cold when the car has sat for a while, and recheck them if you get a significant change in air temperature (summer v winter).

 

[kort]

there is a sticker in the door that has what the pressures should be. a couple of lbs +/- is ok

 

[jadatis]

Need from tires next to yudge the pressures.

1. Maximum load or loadindex
2. Kind of tire to determine the referencepressure.
2 choices here, Standard load AT 36 psi, and XL/reinforced/extraload AT 42 psi.
3, Speedcode, less important. Shal be Q or above fir wich maxload is given for upto 160kmph/99mph.

I make of your story that you have different sizes front and back.
Then rear larger, I think, with higher maxload at same referencepressure, wich could explain the lower needed pressure.

From car please the GVWR and GAWR's, empty weight.

And maximum speed you use, and wont go over for even a minute.

 

[Reg]

By law in California, a dealer/servicer is required to return your car with the pressures set to max GVWR. It’s a legacy of the Firestone/Explorer debacle.

I would set based on what the manual says, though I always err slightly on the high side. Just remember to always set the pressure cold when the car has sat for a while, and recheck them if you get a significant change in air temperature (summer v winter).

thanks.
Doesn't 32 sound low compared to the 38 in the front? I know it is there recommendation but I guess I am trying to understand why such a big difference.

 

[cityhpper]

My car was delivered on the 19" wheels as well (Hankook Ventus S1 evo3 NF0) and I had the same reaction: recommended tire pressure for the rear tires seemed low. I ended up going from the recommended 33 PSI up to 35 PSI for part load conditions. For full load conditions, I made the same 2 PSI addition.

Drove the car about 10 000 km on these wheels, including a three-week road trip across Europe in the middle of summer. Fully loaded car (2 adults + 2 teenagers), Vmax on the Autobahn and several mountain passes. Tires held up remarkably well, very minor and even wear across all four tires.

 

[cometguy]

The label at the bottom of my driver door says 38 psi for both rear and front tires.... Where does it say to use 32 psi for rear tires? The owner manual says to use the numbers on the tire plate on the door sill.

PS... after a little research, I found this:
https://static.nhtsa.gov/odi/tsbs/2022/MC-10217273-0001.pdf

so, basically 33 psi for "partial load" and 37-39 psi for a "full load" on rear 19-inch tires.
Not sure how you quantify "partial" vs. "full" load, and there certainly seems to be a spectrum in between 33 and 39 psi in that regard. My interest stems mostly from wondering what the best pressure is for the least-rapid tread wear. So I guess that the plate on the door sill is for a full load, whatever the heck that is supposed to mean.

 

[Jonathan S.]

I’ve been aiming on the higher side to avoid excessive inner shoulder wear in the rear.
Based upon my recent inspection of tread depth after over 30k, this seems to be working, despite lots of driving in Lowered and some even in Low.

 

[cityhpper]

My CT4 came with the 19" as well. I used the pressure target for "full load" and then added 0,1bar/1,45 psi in addion. Wear was very even across the tyres.

For the 20" and 21" wheels I aim for the "full load" pressures at all times.

 

[paulbroere]

I assume Porsche knows the best pressure for each tire size. I trust them.
I always use the recommended tire pressure + 0,2-0,3bar. (So that I don't have to inflate them again after a month of they lost 0,1-0,2).
I little bit higher is fine, but I wouldn't go 0,5 bar+ over the recommended pressure

 

[Jonathan S.]

But isn’t any tire pressure a compromise among different factors?
Certainly some tire pressures are just wrong.
But for any car model, within some sort of range, different pressures have different pros and cons.

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