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Having a discussion with a co-worker today, about changing tires. I know of the formula for changing the pressure if you use a non OEM tire.
Inflation PSI = Z*X*35%/Y

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


[FONT=Verdana, Arial, Helvetica, sans-serif]But he asked about what if you change sizes. I am not sure what happens. Wondering since the Cruze comes with available 15, - 18 inch tires do they each have there own recommended tire pressure? Maybe folks could post what size tire was stock on their car and what is the recommended pressure from the door sticker. (will look at ours tonight)

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You want to stick as close as you can to the max load and PSI ratings on the tires, then you don't have to worry. Personally I've run either at the next 5 or 10 PSI up from the door placard on all my cars and vans.
 

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Does running at around 10 PSI above the recommended rating affect the health of the tires?
 

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Does running at around 10 PSI above the recommended rating affect the health of the tires?
A great deal of us are 10 or above from the door listed pressures, you just want to stay under that max cold psi on the actual tire.
 

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New to this forum, found this topic searching for tirepressure .
Call myself "Dutch Pigheaded Selfdeclared Tirepressure-specialist, and story began when I got hold of the official European formula and went running with it.
If wanted I will introduce myself in a seperate topic, but dont think its needed.

Your formula probably is intended to use 25% instead of 35% , or you added the 10% for reserve, wich is not that bad.
Further it comes down to lineair calculation, so part of maximum load = part of maximum pressure .
The official European formula has a power in it, and American TRA switched of to that power as late as 2006 for radial tires, but only for P-tires ( in standard load SL, and XL/reinforced/Extraload) and left LT tires to the lower power wich gives more loadcapacity for the same pressure then European used power for all kind of tires ( up to trucktires)

You use maximum cold pressure given on sidewall of tire, but this is not the cold pressure the maximum load is allowed to be carried up to 160km/99m/h, and wich is called reference-pressure in the official formula, but will call it AT-pressure furtheron, because on LT and Trucktires this is given at sidewall only, and not the maximum cold pressure , mostly something like this "maximum load xxxx lbs AT yyy psi(cold).
for SL P-tires this At-pressure is USA system 35 psi, EUR system 36 psi but with exeptions possible.
For XL// P-tires AT-pressure 41psi in USA system , and in EUR system 42 psi again with exeptions.
So those values you need for Z in your formula.

You also assume the GVWR to be devided equally over the 4 tires, and that is not true.
In practice when only driver and a litte load , more weight on front axle then rear, so front higher pressure needed then rear. When fully loaded its the other way around, and you have to highen up the rear pressure but the front can stay about the same ( as a rule of thumb).

then advice given by car-makers is nowadays not based on axleloads anymore, often AT-pressure of tires in America.

For higher speed the official formula gives a system of highening up the AT-pressure depending on speedcode, and as soon as maximum cold pressure is reached , for V,W and Y speedcodes , the maximum load is lowered with a system depending on speedcode again.

Hope you have an idea now, but ask if new questions pup up.
In next post I can give link to my public map of One-drive that belongs to my hotmail.com adress with same username as in this forum ( so jadatis, like this to prefent spamm robots to combine my adress).
Mostly on first posts this is not allowed

Greatings from this Dutch Pigheaded Self-declared Tirepressure-specialist,
( ja dat is ) Peter ( from a Dutch name song from the 1960's, translates as "Yeah thats Peter")
 

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Well that was a noteworthy 1st post, welcome to the forum. Not sure you can post a link this early in post count w/o it being sent to moderation queue. I almost got banned from the ATS AG forums for asking if a part number was correct for a certain model year and linked the GM parts direct link.
 

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I typically go 5psi over the sticker on the door.
I may go a little higher than that if the car/suv is heavy and the side walls bulge more than I like.

Wife's truck recommended pressure by the place we got the tires from is 60psi, side wall is 80psi max I think.. Tires look slightly squishy with a 3.5 horse gooseneck trailer with tack room attached and 3 Amish draft horses in the back. (Don't ask why...)
The suv sticker is 35 but I run it at 40psi since the side walls bulge especially on the front.

The Grand Prix has low profile "racing" tires on it and the door sticker was 32psi. The side walls bulged so bad and felt so soft I went to 40psi. Rides way better and I don't have to worry about rim strikes. Steering was more MUCH decisive.

I just took the Cruze Diesel to 40psi last night. Onboard pressure on the dashboard says 39psi warm. I think I'll add a bit more this weekend so it reads 40psi cold. Man, that is one heavy engine!

Side wall pressure rating for the Grand Prix, Vue, and Cruze all say 44 or 45 psi so 40psi is fine.

Tires warm up going down the road and depending on the starting temp you can see a 5psi or difference between cold and warm.

The only cars I have ever had at the sticker pressure was the S-10 on the rears (fronts were 40psi) with an empty box, and Tempo which was a pretty light car.

What I would do is look at the tire and sidewalls vertically and again from about 10 feet away facing the tread and see how much bulge you have when cold. If the bulge is more than 1/2 an inch on each side compared to the rest of the sidewall add some more air.

Often the rear is fine but the front could some more pressure with the weight of the engine.
Checking your cold tire pressure is much important now as we go into winter with wild temperature swings!
 

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The problem with the door placard pressures listed in the US is that Americans are treated as if they're all the Princess and the Pea when it comes to vehicle ride. The pressures listed too low for handling performance and in at least one case (Ford Explorer) so low the tire itself became prone to blowouts from overloading. Adding a few PSI extra will keep the bulk of the ride softness while giving the driver a much more consistent handling feel.
 

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Can't say I've ever once looked to the door sticker for inflation pressure.

I just generally base it off what I know of similar tires, the use for the car, and the maximum psi on the tire and go from there.
 

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Can't say I've ever once looked to the door sticker for inflation pressure.

I just generally base it off what I know of similar tires, the use for the car, and the maximum psi on the tire and go from there.
When you do manage to glance at it, Let me know what it says for the spare tire.....
 

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pfft ha ha!!! :D

Go by the MAX on the sidewall of the spare if you have one especially if it is a doughnut spare.
Be sure to check it once a year.
I don't have a spare either! :grim:
I have a spare and have checked it exactly one time - when I put it on my car. It was low so I simply inflated it to max sidewall PSI before I started driving again. Everyone, and I mean EVERYONE, should have a 12v air pump in their trunk at all times. I've used the pump far more than my spare tires.
 

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I use one of those emergency jumper batteries with the pump built in. its a little slow but it works and seems to hold a good charge for a couple months.

I like that the cruze has one in the boot.

If I ever do a full emissions delete I'm def adding in a spare.
 

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Hmm makes me wonder if that's why my new Cruze handles like crap compared to my old Cruze with the same 18x8.5 245/40 rims/tires... I ran them at 30-35 PSI on the 2012 Cruze and 35 on the new 2016 Cruze but **** does the steering suck! I'll try bumping it to 40 PSI and see what it feels like
 
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