Correct Tire Pressure after aftermarket wheel install....

[Mark B]

Hey Everyone,
My car originally had 16x7" wheels on it. A couple months ago I upgraded those to a set of 17x8's What tire pressure should I be running now? Shoud I keep the same pressure as suggested by Pontiac. When I first had them put on, they looked good so I didnt even worry about it. However I did a tire rotation last weekend and now it looks like the fronts are low due to the increased weight!!


Thanks in Advance

[Christopherrr]

wondered that myself...but I run stock rollers. If I'm not mistaken, you should always run suggested PSI.

[PaganEgyptian]

Being a mechanic in which I have went through several tire manufactures classes, it all depends on tire contruction. I am thinking Pontiac specs 30/30 psi for the car, but changing the load capacity of a tire will effect the wear pattern at a certain PSI. To achieve good wear since moving to a Conti-tire(great choice), I recommend 35f/35r to 38f/35r psi, depending on your driving habbits. It's a formula I use. But I will save your time.

I upgraded from 245/50R16 to 275/40R17 which yes is bigger, but the it will not hold as much weight, so I had to compensate the difference. Oh, I took a 30/30 setup and rolled the tire off the rim.-plus 35PSI in 16"and17" V-6-birds will make it wear better than 30PSI and will keep it on the rim better. I did that at the Police driver training school.

[N20LT4Bird]

Originally posted by PaganEgyptian
Being a mechanic in which I have went through several tire manufactures classes, it all depends on tire contruction. . It's a formula I use. But I will save your time.

WOW! A formula for tire pressure. Pretty high tech. I kick the tire with my shoe.

[Mark B]

Very cool, thanks for the reply. I will get the correct amount of pressure in my tires tomorrow. I KNOW THAT THEY ARE NO WHERE NEAR WHAT YOU SUGGESTED. Does it matter the that the outside tire diameter changed? If I am correct, I think these tires are off by 3%.... Or 1" shorter than the stockers. The stock size was 225/60-16.

[PaganEgyptian]

with a 225/60/16 tire that has a load index of 96, if you went to 17's, depending on size.....it can vary.
245/45/17 would require around 38 psi. most tires in that range will hold up to 44-51 psi.
I would only run that kind of pressure in the fronts as that is where the engine is and takes most of the weight.
add 2-3 psi for aggressive driving.

I know a pet peeve of mine is seeing these great cars with nice aftermaket wheel/tires that are so underinflated that it wear on the inside and outside. Even the tire store I worked at could'nt figure that on out untill I explained it to them.

And even some salesman sold Michelin Symmetry tires to a girl with a 97 firebird.
Can you say shoulder wear, since those were originally designed for cadillac.
Ride, not performance especially stiff suspensions....

[wolfdogs]

Originally posted by PaganEgyptian
Being a mechanic in which I have went through several tire manufactures classes, it all depends on tire contruction.


not true. its dependent on size. all tires must meet the requirements of the Tire & Rim Yearbook which inlcude demensions and load carrying capacity. KenC



I am thinking Pontiac specs 30/30 psi for the car, but changing the load capacity of a tire will effect the wear pattern at a certain PSI.
To achieve good wear since moving to a Conti-tire(great choice), I recommend 35f/35r to 38f/35r psi, depending on your driving habbits. It's a formula I use. But I will save your time.

thats too high of a pressure. it reduces the contact patch and affects handling, both thru the size of the contact patch and the flex of the cord body which affects the traction thru the tread pattern and wear. kenC

I upgraded from 245/50R16 to 275/40R17 which yes is bigger, but the it will not hold as much weight, so I had to compensate the difference.


Excuse me? Why are you giving this false information? 245/50r16 carries 1356lbs at 26 psi, 1510lbs at 32psi, and 1576lbs at 35 psi. a 275/40r17 carries 1371lbs at 26 psi and 1653lbs at 35psi. It carries MORE weight! KenC

Oh, I took a 30/30 setup and rolled the tire off the rim.-

Sorry, I dont believe this. ..just how did you de-bead the tire? KenC

plus 35PSI in 16"and17" V-6-birds will make it wear better than 30PSI and will keep it on the rim better.

it will cause pre-mature center crown wear and reduce traction. Tire dont de-bead (come off the rim)..that happens very rarely..and only at extreme low inflation pressures and hard cornering applications. folks drop down to 22 psi to drag race on the rear and tire does not even spin inside the rim. this is not an issue whatsoever. KenC

I did that at the Police driver training school.

did what? I am very familiar with law enforcement training center drivers school operations. i set up the blow out course at the FBI Acadamy in Quantico. Im curious as to what you are refering to..... KenC








Ken C
Manager, Federal Tire Qualification Program
U.S. Govt.
(yes.......im a tire engineer


just keeping the records straight. no flames intended. but false info can hurt fellow f body enthusiasts.

[wolfdogs]

give me the tire size, thats what dictates the pressure, not the rim size. thanks.

[wolfdogs]

Originally posted by Mark B
Very cool, thanks for the reply. I will get the correct amount of pressure in my tires tomorrow. I KNOW THAT THEY ARE NO WHERE NEAR WHAT YOU SUGGESTED. Does it matter the that the outside tire diameter changed? If I am correct, I think these tires are off by 3%.... Or 1" shorter than the stockers. The stock size was 225/60-16.

it will change your speedometer reading.

please read my post to your friends input on pressure. he is not correct.

[Rob B (shoebox)]

Originally posted by wolfman
...


just keeping the records straight. no flames intended. but false info can hurt fellow f body enthusiasts.

I agree, Ken. Most of that "advice" seems pretty poor.

[Mark B]

Yeah I know about the speedo thing. That doesnt bother me, but the fact that my odometer is also adding extra milage kinda sucks. If the numbers that I was given before were off... What pressure should I run? Remember I went from a 225/60-16 to a 245/45-17. Stock tire pressure was 30/30.

[Jeff_in_Atl]

I was staying away from this thread but I would start with 32 front and 30 rear. My gut feeling is anything over 32 is going to wear in the center before the outside. Also a higher pressure makes the car feel twitchy on the freeway.

[wolfdogs]

Originally posted by Mark B
Yeah I know about the speedo thing. That doesnt bother me, but the fact that my odometer is also adding extra milage kinda sucks. If the numbers that I was given before were off... What pressure should I run? Remember I went from a 225/60-16 to a 245/45-17. Stock tire pressure was 30/30.

your new tires are a little less than 1" as tall as your stock ones.

at 29 psi, which is 1 psi off stock running pressure ..your o.e. tire caries 1455lbs. your new tire carries 1351lbs at 29 psi. and 1437lbs at 32psi.

so.........run 33 to 34 psi in them. monitor the wear patter in across the tread face for about a 1,000 miles. because of the lower aspect ratio and stiffer sidewalls, the contact patch wont spread during sidewall flex like it does on your o.e. tire. if you notice its wearing fast in the center...drop down 2 to 3 psi and monitor another 1000 miles. being that you are front wheel drive, be sure and rotate them at around 6000 miles..front to back is fine.

[N20LT4Bird]

Originally posted by N20LT4Bird
WOW! A formula for tire pressure. Pretty high tech. I kick the tire with my shoe.

I was trying to be a little more subtle, but wolfans debunking works too. Ouch!

"Tire manufacturers classes" = watched a video during training at Wal Mart's tire center.

[Jeff_in_Atl]

Yea, what Wolman says. (I didn't have time to type all of it.)