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Tire pressure question

Sup everyone,
Been awhile since I've posted here. Just been lurking around.
I recently purchased new rims for my car off of eBay and was wondering how I would determine the proper tire pressure for the new tires.
My old tires were 215/60 R15 Front & Rear.
Now they are 205/50 R17 Front and 255/40 R17 Rear.
Do the tire pressures stay the same?
Also, is it inaccurate to measure tire pressure prior to mounting?
Thanks in advance for the advice.
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Old 07-28-2004, 09:43 AM
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not sure.

look on the side of the tire and see what the tire mfg. gives for the max.

porsche has their numbers for the 215/60's, which mine came with, not sure on the later models.

hey diddle-diddle pick something in the middle and try to keep the nose in front.
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Old 07-28-2004, 04:20 PM
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Quote:
Originally posted by bearone2
not sure.

look on the side of the tire and see what the tire mfg. gives for the max.
inside? unfortunately the tires are already mounted to the rim
the rim and tires were OEM off a 996. you think i should find out the original tire pressure for the 996 and use those numbers?
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Old 07-28-2004, 05:40 PM
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no, outside.

sure if the weights are about the same.

i'm not sure if the wt distribution is the same.
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87 944S, alpine white, 5sp died a violent death
84 944, silver/brown, auto, gone but not forgotten

"may the force be with you"
Old 07-28-2004, 06:39 PM
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The tires maximum inflation pressure has nothing to do with the pressure you want for your car. The 996 will probably have different tire pressures due to different weight on all wheels. You should use the same pressure as you did on the old wheels as it is the weight on the wheel that is essential for deciding tire pressure. With fine tuning of handling characteristics being the second factor. I have never heard about lower profile tires needing more pressure than high profile tires.

The pressure should rise a bit when you mount the tires and put weight on them, but I am not sure how much. It could be minimal.

Edit: Added second factor and rising pressure.

Regards:
Eirik Kvello-Aune
www.944968.com

Last edited by mmmbeer; 07-28-2004 at 06:53 PM..
Old 07-28-2004, 06:47 PM
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http://www.garageworks.741.com/Stupid.htm
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Old 07-28-2004, 08:58 PM
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35psi front and rear..thats what I usually use when I have tires that say do not inflate above 40 psi to seat beads. For the lower speed passenger tires that say do not fill beyond 35 psi to seat beads, I usually fill them with 30. Trucks get 32 psi, including ones with 24" rims. Anything lower eats the tire, anything more causes blowouts...
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Old 07-28-2004, 09:03 PM
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Quote:
Originally posted by gearhead290
http://www.garageworks.741.com/Stupid.htm
Nice argumentation!
What exactly is stupid?

Regards:
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www.944968.com
Old 07-29-2004, 05:21 AM
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Quote:
Originally posted by mmmbeer
The tires maximum inflation pressure has nothing to do with the pressure you want for your car. The 996 will probably have different tire pressures due to different weight on all wheels. You should use the same pressure as you did on the old wheels as it is the weight on the wheel that is essential for deciding tire pressure. With fine tuning of handling characteristics being the second factor. I have never heard about lower profile tires needing more pressure than high profile tires.

The pressure should rise a bit when you mount the tires and put weight on them, but I am not sure how much. It could be minimal.

Edit: Added second factor and rising pressure.

Regards:
Eirik Kvello-Aune
www.944968.com
thank you a sensible answer at last
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Old 07-29-2004, 05:29 AM
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when the tires are cold, 29 front, 36 rear.
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Old 07-29-2004, 05:43 AM
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Quote:
Originally posted by CJFusco
when the tires are cold, 29 front, 36 rear.
i know that as being the stock tire pressure for stock tires size but i guess i assumed with a wider contact patch, you would want a proportionally greater tire pressure.
here's a question:
what is the tire pressure for a 944 turbo or 968 who's stock tire widths are greater than a stock tire width of a 944 NA?
thanks for all the advice
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Old 07-30-2004, 10:09 AM
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I've asked this same question before, and have really never been able to get a sound answer from anyone. How do you recalculate the correct tire pressure for non-stock sized tires.

The following is how i've always viewed it. Feel free to argue, i don't know if this is right.

A wider tire should have a lower pressure, not higher. In theory, the contact patch square area should be exactly equal to the weight applied to the tire divided by the tire pressure. That is because the weight of the car is balanced by the internal air pressure against the contact patch. Remember pressure is pounds per SQUARE INCH. Each square inch of tire contact (inflated to 29psi) is capable of supporting 29 lbs. In practical experience, it's not exactly this, because the tire has some internal spring rate to itself, but we'll ignore this fact for the sake of this discussion. For example, if a tire (any sized tire) has 700lbs vertical load applied, and inflated to 29psi, there will be 24.14 sq. in. of contact area for that particular combination. This contact patch is approximately square in nature. The width of this patch is fixed for a particular sized tire, set by the tire width, and the length (in the direction of rotation) is set by the pressure. Each tire has an optimum contact patch area. That is why car manufacturers reccomend higher inflation pressures for a fully loaded vehicle. This keeps the contact patch approximately the same. Unfortunately, we do not know the optimum contact patch area for the tires in question. You could try calling the tire manufacturer, but your request is likely to be met by someone on the other end of the line going "HUH?" If you can actually get through to someone in an engineering position, they could tell you the optimum contact patch area for this tire and weight combination. (plus they should also be better able to explain how to calculate tire pressures.)



215 is the reccomended width for the 944 tires. You have 205's in the front, and 255's in the rear. The outer diameter of the new tires is approximately the same as the original tires. Tire aspect ratio and wheel diameter mean nothing to the pressure calculation. Everything is based on the outside diameter and width of the tire. The way i would calculate the pressure is as follows. Using the stock tire width of 205 and an estimated weight of 700lb on each tire (the actual weight does not matter since everything is proportional) the front contact patch area
is 24.14 sq. in. (@29 psi) and the rear area is 19.44 sq. in. (@36 psi). Using the stock tire width of 8.46" (215mm) the front contact patch would be 2.85" from front to back. (8.46" x 2.85" = 24.14 sq. in.) and the rear contact patch would be 2.30" (8.46" x 2.30" = 19.44 sq. in.). Here is where my theory is questionable. I will make the significant assumption that we should keep the front to back patch length constant. 2.85" for the front and 2.30" for the rear. Using the new front tire width of 8.07" (205mm) times the 2.85" length gives an area of 22.99 sq. in. and the rears 10.04" (255mm) x 2.30" = 23.09 sq. in. Using the same 700 lbs, divide by the contact patch area to get the required pressure to maintain that contact patch area. For the fronts, 700lbs / 22.99 sq. in. = 30.44 psi and the rears 700 / 23.09 = 30.31psi. As you can see, as the tire gets wider, the pressure goes down. The rear pressure went down from stock and vice-versa, the front pressure went up from stock because of the narrower replacement tire.

If these tires came off a vehicle that you can identify (perhaps a boxster, or similar) you could use the weight of the original vehicle on that particular wheel, and the original vehicle tire pressure on that wheel to get an area, then just adjust for the weight of the vehicle that you're installing them on to keep the same contact patch area, assuming that the original vehicle contact patch area is optimum for that tire. (just for example, 900lbs / 32 psi = 28.125 sq in. for the original car, 700 lbs (your car, but here the exact weight does matter) divided by 28.125 = 24.88 psi.)

Of course, always adjust based on handling.

Again, feel free to argue, that is only my theory on tire pressure calculations. I would really like to have someone from a tire company, or someone who does this kind of thing for an automaker to chime in and give us a real answer.

Mike

P.S. hope i didn't loose too many people in the calculations

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Last edited by mike944; 07-30-2004 at 12:47 PM..
Old 07-30-2004, 12:45 PM
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whoa there, mike

try it with the stock recommendation of 36psi, and then mike's recommendation of 30-31... see which "feels" better (for some reason I can always tell when my car needs air or is over-inflated by feel). I bet mike's calculations are accurate. He's a smart guy.
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Old 07-30-2004, 01:48 PM
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Why not just chalk the tires? If the chalk wears off the sides, add more air?

Works in racing, why not on the street?
Old 07-30-2004, 04:31 PM
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KEYWORDS: HOLYCRAP, THE LONGEST AND MOST DETAILED EXPLANATION ABOUT TIRE PRESSURE EVER

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Old 07-30-2004, 04:53 PM
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Old 07-30-2004, 06:15 PM
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mike, what would be the correct air pressure for 225/60/15s on a 944, front and rear

(not good at math)
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Old 07-30-2004, 09:55 PM
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mike,
thanks for the thorough info. the rims are 2002+ 996 wheels. i was considering finding out the 996 weight, weight distribution and tire pressure and figuring out my tire pressure from there.
if i do, i will post my results and you can check my math, too
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Old 08-04-2004, 07:42 AM
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