Tire pressure and low profile tires

[Quickstep192]

I searched for earlier posts on this and came up empty. I also searched the web and got conflicting answers.

When switching from stock wheels larger wheels and lower profile tires, do you change the tire pressure, or just go with what's on the door jamb sticker?

[HarryD]

I went from 15 to 16 inch tires. No change in pressures. Seems to be ok in terms of wear and handling.

[dreichman]

Pressure is pressure, no need to change. In other words. 30 psi is 30 psi. It does not matter what shape or size of the container.

[stownsen914]

Standard pressure is a decent place to start. Any variation is most likely to be due to the tire. Check with the tire manufacturer what they recommend for that tire. If the car in question is the one listed in your sig (a '73), a lot has changed re: tire technology in the last 50 years.

[darrin]

Originally Posted by Quickstep192

I searched for earlier posts on this and came up empty. I also searched the web and got conflicting answers.

When switching from stock wheels larger wheels and lower profile tires, do you change the tire pressure, or just go with what's on the door jamb sticker?

what tires/wheels are you changing to? If a stock size for a later/different 911, I'd go with the pressures Porsche specified as your starting point.

[ADDvanced]

I usually run higher than normal pressures in everything I own. Reduced rolling resistence on the hgihway and less rolling/wearing/scuffing off the corners/sidewalls in corners.

[Otter74]

Your car weighs the same with either set of tires, tire type is the same (e.g. radial), and tire volume is the same, which means that on a first and second order pressures will remain the same.

[darrin]

Originally Posted by ADDvanced

I usually run higher than normal pressures in everything I own. Reduced rolling resistence on the hgihway and less rolling/wearing/scuffing off the corners/sidewalls in corners.

By how much do you overinflated? I'd be concerned that overinflation could cause excessive wear down the middle of the tire

[ADDvanced]

idk, 5-10 PSI. You get more wear in the center, yes, but if you corner hard you wind up scrubbing the corners, esp if you drop down to 30psi for driving for twisties...

Works for me, but YMMV

[VWLoosenuts]

Run what the recommended tire pressure says on the tire.

[icarp]

Q , I sent you a PM, There are many variables to this , too many to type .
I'm happy to talk on the phone , then you can type what you decide .
Ian

[Bill Douglas]

Try 34 front and 36 rear, and see how you get on.

If you don't like it or a rattle has started inside the car try 32 front and 34 rear. Another thing to note is most gas station air pumps seem to be calibrated differently. My tire place gets theirs calibrated regularly.

[gptom]

I just added Pirelli P4 195/65 15 on 6J wheels. they feel like cement overshoes at 29F 32R. Is this ust the nature of the tire?

[chrisbalich]

I'm running 225/40 & 245/40 on 17" wheels this year. I'm running them at 30psi all around right now.
Last year, I was on 215/40 up front and didn't run them below 32psi. They understeered and wore poorly.
My car is ~2250lbs wet and driven like I have sense.

What I'm getting at, is tire pressure is highly specific to your car & needs. A light car will need less pressure than a heavy car. Stiffer sidewalls and high tire pressure can be harsh. Slicks run less pressure than radials. If your roads are glass, you can run less pressure and get more grip (to a point) or you can pump em up hard as rocks and focus on fuel economy. Both affect the contact patch which affects traction and tire wear.

You've got to start somewhere and tinker with your pressures until you get what you're after. The manufacturer's specs are as good a place to start as any.

[Tremelune]

Originally Posted by VWLoosenuts

Run what the recommended tire pressure says on the tire.

You sure? I've only ever seen a maximum pressure specified on the tire itself.

[icarp]

There is so much to write on this subject it should be a multi chapter book.
for example 1975 911 3.0 205 hp fronts 205/55/16 and 225/50/16 rears . First set of tires ,
Toyo Proxes r888 , track days and canyon fun, "alignment is all important " custom for every set of tires, some do cross pollenate . 31 square COLD ! because the hot pressure is the important pressure . Ten minutes into the drive , check for hot pressures , looking for 36psi front and 37psi rear. that is for the r888. 10,000 miles later we tried a new set of tires. Firestone indy 500
we added .25 degree more negative camber all around. Basically the same kind of tire , so we went for the same pressures . 31 COLD , the car was fun ! predictable . just right for the same kind of driving . Then one day I took the car for a test run up the Angeles Crest Hiway, I made a change to the hot pressure , 41 front and 42 rear, oh my god what a difference ! As I said , this needs to be it's own book . I ran 40,000 miles on my RS clone , 60 series mini van tires , proper alignment and I had a wear pattern of not more than .015"difference from the inside tread to the outside of the tread.
This was canyon fun and teaching 8 private track days , the track was just for me and my two students, hiway miles of comfort , getting food and doing errands . All at 30 psi square cold and 34 hot front and 35 rear hot. So while tires seem similar they really are different . It is best to start with a factory recommended PSI you must follow up with hot psi and try a different psi now and then , it is not often that a tire wants to be more than 37 psi, although 30 series tires on my turbo like 39 font and 40 rear hot .
Side wall stiffness, tread depth, softer compounds , racing on a track, they all can make a difference to what the tire wants for pressure . DO NOT FORGET ALIGNMENT .

Some of the smaller tired 911's, 205 and under, 2200lb cars like to start at 28 psi and hot at 32 and 33 psi hot

If you would like to have a phone conversation about tire pressures send me a PM
Just my thoughts
Ian