Tire pressure??????

BlwnGazkit · Aug 28, 2007

Ok, so I'm running the new 235/55ZR17's and not sure what pressure to run them at for general road driving.

Max pressure on the sidewall is 51 psi with a 1600lbs load. So I guess I'm gonna run them at about 48psi unless you all think differently.

Lemme know!

Soaring1 · Aug 28, 2007

BlwnGazkit said:
Ok, so I'm running the new 235/55ZR17's and not sure what pressure to run them at for general road driving.

Max pressure on the sidewall is 51 psi with a 1600lbs load. So I guess I'm gonna run them at about 48psi unless you all think differently.

Lemme know!

Don't pay any attention to that Max pressure thing. That is the maximum pressure it can take before it blows up.
I run my tires at anywhere between 30 and 32 psi. It depends on the tire.

rbohm · Aug 28, 2007

48 sounds like a good starting place. you want enough pressure to have the tread sit flat on the road, but not too much so as to avoid an excessively stiff ride.

BlwnGazkit · Aug 28, 2007

Ok, so I have to completely opposiing answers here....

Now 32psi sounds like what would be for a stock tire, but then a stock tire was made 40 years ago with 40 year old technology. Seems to me that a larger AND newer tire should be able to handle and maybe even require the high pressures...

Not sure though, just spouting nonsense and hoping for it to sound good!!

Soaring1 · Aug 28, 2007

What running tire pressure does the tire company suggest you run them? Again, that maximum tire pressure is just so they don't get sued if you blow your head off by overfilling them. I don't know of any tire, no matter who makes it, or whether or not it is a low rider or not that runs anything over 32 pounds on a mustang.

BlwnGazkit · Aug 28, 2007

I'm gonna contact Pirelli tomorrow, but it probably will be about 32psi or so...

Hmm, what do 05+ Mustangs weigh? It would seem that the running psi is a function of GVW and max tire load.

Soaring1 · Aug 28, 2007

BlwnGazkit said:
Ok, so I have to completely opposiing answers here....

Now 32psi sounds like what would be for a stock tire, but then a stock tire was made 40 years ago with 40 year old technology. Seems to me that a larger AND newer tire should be able to handle and maybe even require the high pressures...

Not sure though, just spouting nonsense and hoping for it to sound good!!

The technology has not changed in 40 years. Tires and wheels are still made the same today as they were 40 or so years ago except the tire companies went from biased ply to radial plyl. But, all laws still apply. It doesn't matter how big the space, psi is still psi. It was psi back in 1965, and it is still psi in 2007, soon to be 2008. If you fill those tires passed 32 psi, you are ignorant to the laws of pressure as compared to weight.

BlwnGazkit · Aug 28, 2007

Ok, I think you miss understood what I was referring to as having changed.

Tire compounds have changed as well as how they are produced and manufactured (atleast the performance ones). Laws of Physics do not change. Correct, and I understand them to a relative degree.

What does change is how the physics of a tire and it's manufacture are understood and applied.

10secgoal · Aug 28, 2007

rbohm said:
48 sounds like a good starting place. you want enough pressure to have the tread sit flat on the road, but not too much so as to avoid an excessively stiff ride.

Pirelli is ,ore than likely going to tell you this. Each tire can vary quite a bit. That's why you always read the side. If you go over 51, it won't blow. I run 65 in a bug tire.

Soaring1 · Aug 28, 2007

10secgoal said:
Pirelli is ,ore than likely going to tell you this. Each tire can vary quite a bit. That's why you always read the side. If you go over 51, it won't blow. I run 65 in a bug tire.

65 pounds in a VW bug tire, huh? Now why would you do that?

TD68302 · Aug 29, 2007

I would go about 30 - 32psi, and that might by a little hi

Soaring1 · Aug 29, 2007

Yeah, most tires say to run 28. Back in the day, the tire companies said to run 24. That was too little. 30-32 is a good pressure to run on modern tires so they last. The guys who run more than that will see the center threads wearing out before the rest of the tire. The ones who run less than that will see the edges wearing out before the rest of the tire. Been there, done that.

BlwnGazkit · Aug 29, 2007

Just and FYI, I kicked the pressure up to 44psi and saw a notable increase in my mpg....

I'm just gonna run them like this and measure the tread after a bit and see how everything's wearing and make adjustments then.

Thanks for the advice guys.

68EFIvert · Aug 29, 2007

BlwnGazkit said:
Just and FYI, I kicked the pressure up to 44psi and saw a notable increase in my mpg....

I'm just gonna run them like this and measure the tread after a bit and see how everything's wearing and make adjustments then.

Thanks for the advice guys.

I have been told by tire guys to run them about 50 psi. I felt that was extreme so I only have about 42 in mine. Odd thing about it is I sent an email to BFG 2 days ago to get their opinion on tire pressure. I will post back when I hear from them.

CraigMBA · Aug 29, 2007

I used to work for BFS. I'll bet that they (BFG or Perelli) won't respond because of liability.

Air (pressure and volume) is used to offset load capacity. This is why the tires on those Explorers failed. They were underinflated for the application , or were too small. There is a reason trucks run 90-110 pounds per tire.

Your owners manual (door tag on a new car) states 28 pounds. The air chamber is about the same on the 17" tire (it's actually a little bigger) so you can safely run 28 to start. For OT applications I'm probablly starting at 40.

If you want to run your tire at the sidewall max and have a car that rides poorly, has poor tire wear, and less than optimal traction, go for it.

68EFIvert · Aug 29, 2007

I have the answer. I called BFG because their email response was not sufficient. The email basically told me to use the tire pressure in the owners manual. To give you a little background on my setup I am running 245/45/17 up front and 255/40/18 in back. The woman put me on hold while she ran her calculations to took into considerations original tire pressure, volume of air and converted that into my new size. Long story short 30psi front and rear. If you have a big block I would assume you would run more than that, perhaps 32-33 psi. I hope this helps.

CraigMBA · Aug 29, 2007

What she did was look up the OE pressures and figure out you had a larger than stock air chamber and then bump it up to make sure it was OK.

You could likley run as low at 23 pounds and still be OK, although I don't know why you would want to.

Where the pressures get really important is on these monster SUV's with razor thin 22" or bigger tires that really aren't load rated for the weight they are carrying.

grego37 · Aug 29, 2007

good topic:

BlwnGazkit · Aug 29, 2007

Then can anyone explain the increase in gas mileage from upping th PSI to 44?

Yes, more PSI in a tire gives it the capability to support more. But correct tire pressure is a factor of vehicle weight, tire construction, and size. Tires with stiffer sidewalls require LESS pressure to support the same load as a tire with a softer sidewall. The idea is to have a tire that uses the air pressure AND sidewall to evenly support the tread giving it an even wear pattern.

For example: A soft sided Space-Saver-Spare tire may require 50psi to support 1200lbs even across the tread. A standard road tire would require less since the sidewalls are tougher. The amount less would be determined by the sidewalls strength, the treads rubber compound and how wide the tread is. The wider the tread, the higher the pressure needs to be to support the center.

I started this thread, not to argue about what the proper inflation should be, but instead to see if anyone had any prior experience with a good pressure on this size tire and vehicle weight combo. I have a fairly good experience with motorcycle tires and how they should be inflated for racing applications. The proper inflation is NOT determined by what's on the door of a car unless you keep using the same brand and size of tire. If you switch tires you're switching to a design that the manufacturer never ran any calculations for and the psi rating may not apply anymore.

In the past we (my bike racing team) have always started a new tire with a simple formula and using the vehicle's weight and info printed right on the tire.

You take the GVWR and divide it by the number of tires. Then you dived that by the maximum load rating of the tire at it's maximum pressure. This gives you a percentage which you then multiply by the maximum pressure stated on the tire.

Generally this gives you a minimum pressure to use as a starting point; the final psi is somwhere close and usually a tad higher. HOWEVER, in my case with these Pirellis I noticed a decrease in performance and I wanted to see if anyone had a better pressure to run. FYI, using that method, the Pirelli tires would be inflated to 30.2 psi.

CraigMBA · Aug 29, 2007

grego37 said:
Just dont ask the guys at Marlos frame and alignment. They told me you allways run your tire pressure at the max. pressure sidewall rating.

Without directly commenting on Marlo's, have you noticed that most of the guys putting tires on don't have PHD's in Mechanical Engeneering?

It's pretty standard in the tire business to do that. You'll get in real trouble for underinflating a customers tire, but if you pump it up to the max sidewall rating, nobody will bitch.

Tire pressure??????

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