Tire Pressure

wantin150 · Mar 22, 2006

"Not that you drive 100MPH of course..."

Who me? No, not often anyway, anymore. I will still run my Matrix to its limiter at 138 every now and then. I used to frequently operate emergency vehicles at speeds up to 120 mph, and even 150 mph once. I kept the tires at my +3 principle and never had an issue.

mechtech · Mar 23, 2006

Most people look at the sidewall tire pressure, and think it's what should be used.
It is a rating with pressure AND weight given.
Nobody finishes the sentence and reads the whole statement. They only see the pressure.
That pressure is only correct for the high load embossed on the tire. It is usually a much higher weight than what that corner of the car is loaded to.

CapriRacer · Mar 23, 2006

I am going to slightly disagree with mechtech:

If the sidewall of the tire says something like:

Maximum Load XXXX at YY pressure

then I agree.

But if the sidewall says something like:

Maximum Load XXX
Maximum pressure YY

then the pressure and the load do not correspond.

For example a P215/60R15 has a maximum load of 1411 pounds and that occurs at 35 psi. Use a lower pressure and the load is reduced as well.

However, the sidewall of a P215/60R15 can say one of 3 pressures: 35 psi, 44 psi, or 51 psi. Regardless of which pressure is listed the maximum load is 1411 pounds and that occurs at 35 psi.

As mechtech said: "Nobody finishes the sentence and reads the whole statement."

CapriRacer · Mar 25, 2006

quote:

Originally posted by George Bynum:
I've trimmed the OP post.

Originally posted by CapriRacer:
For example a P215/60R15 has a maximum load of 1411 pounds and that occurs at 35 psi. Use a lower pressure and the load is reduced as well.

However, the sidewall of a P215/60R15 can say one of 3 pressures: 35 psi, 44 psi, or 51 psi. Regardless of which pressure is listed the maximum load is 1411 pounds and that occurs at 35 psi.

Click to expand...

.....Web research shows they have 2 different tires of this size, one with a P, one without. The ratings differ.....how can the same "size" have different service descriptions?... [/QUOTE}

There are several ways:

1) Extra Load va Standard Load. This is where a tire uses more inflation pressure to carry more load. Typically an XL tire will use 41 psi.

2) Different standards organiztions - There are 3 major one - TRA (Tire and Rim Association = US) ETRTO = European Tire and Rim Technical Organization (Europe), and JATMA = Japanese Automotive Tire manufacturers Association.

While these organix=zations have slightly different syandards for load and infltion they are pretty close.

For example a P215/65RR16 (TRA) has a 95 Load Index (1510 pounds at 35 psi) and a 215/65R16 (ETRTO) has a Load Index of 98 (750 kg at 2.5 bar = 1653 pounds at 36 psi). If you poltted the 2 curves, you would see they are pretty close, but they are ever so slightly different.

hope this helps.

alreadygone · Mar 25, 2006

I thought the "P" stood for passenger, non "P" (blank) is either a suv or light truck grade, although light truck should have "LT" in front of size and usually denotes either load range C, D, or E. Extra load tires have "XL" behind size.

Bob

George Bynum · Mar 25, 2006

I've trimmed the OP post.

quote:

Originally posted by CapriRacer:
For example a P215/60R15 has a maximum load of 1411 pounds and that occurs at 35 psi. Use a lower pressure and the load is reduced as well.

However, the sidewall of a P215/60R15 can say one of 3 pressures: 35 psi, 44 psi, or 51 psi. Regardless of which pressure is listed the maximum load is 1411 pounds and that occurs at 35 psi.

I happen to run 215/65R16 on my vehicle. When I bought the last set of tires, I decided on the BFG Traction TA. Web research shows they have 2 different tires of this size, one with a P, one without. The ratings differ. The higher rated one is unidirectional; the P one is not. These are taken directly from manufacturer literature; how can the same "size" have different service descriptions? Note; the P one is a Ford spec tire per the literature. Is that enough said?

Size, 215/65R16, P215/65R16
SvcDsc, 98T, 96T
Sidewall, BSW, BSW
MSPN, 46501, 81499
Stk #, 122-862, 516-170
Rims, 6.0 - 7.5, 6.0 - 7.5
Section, 8.7 on 6.5, 8.8 on 6.5
OD, 27.0, 26.8
Tread, 10.0, 9.5
RPMile, 770, 776
Max Ld, 1653@44, 1565@44

CapriRacer · Mar 25, 2006

quote:

Originally posted by alreadygone:
I thought the "P" stood for passenger...

Yes, that's right. Those are called P metric tires.

quote:

...non "P" (blank) is either a suv or light truck grade....

Sorry, that's not right. The lack of a "P" or"LT" merely means the tire is a tire using the ETRTO based standard. We sometimes called these tires Euro metric.

quote:

....although light truck should have "LT" in front of size and usually denotes either load range C, D, or E. Extra load tires have "XL" behind size.

Bob

And the last part is correct.

This is a bit confusing, but you have to avoid thinking about vehicles and orient yourself to think about tires, and load standards.

The TRA uses letters to help designate what standard is being used and this usually identifies a vehicle usage type. For example: a "P" is a Passenger car, an "LT" is a Light Truck, "ST" is Special Trailer, an "NHS" is Not for Highway Service, "HM" is Mobile Home, and many more!

This is something only the vehicle designer should not worry about. The consumer should worry about getting the proper "size", which will be listed on the vehicle placard and the "size" will include the letters. The only exception to that rule is the lack of letters, which is OK for passenger car tires.

I'm sure some of you have noticed that light duty pickups and SUV's generally use P metric tires. and that heavy duty versions use LT metric tires. This has everything to do with load capacity and nothing to do vehicle type.

I could go on, but hopefully, this will clear up this bit of confusion.

STSinNYC · Aug 18, 2006

We kept the MXV4+ tires on our 02 Accord V6 inflated to 40 PSI, max. cold pressure was 44. Honda label reco'd. 30. We rotated every 6-8K miles. At 69.4 K miles, the tires still had enough tread left to pass inspection. The tires wore evenly, the middle areas did not show more wear than the outer areas. Fuel economy was better and the car handled very well.

Fuel is expensive and won't be getting cheaper any time soon. Makes sense to inflate the tires well, whatever you think that is for you.

Tire Pressure

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