Load Ratings and PSI

[Gene K]

I keep running into people that obviously dont understand how modern radial tires for passenger cars and light trucks are rated. They see maximum load and maximum psi on the sidewall and think they have to correspond. This is not always true. The rules body for this in the USA is the Tire and Rim Association (TRA).

Pressure at which maximum capacity is rated:

Passenger Car (P)
SL - 35 PSI
XL - 41 PSI

Note: When used on vehicles classified as trucks passenger car weight ratings must be reduced 9.1%.

Light Truck (LT)
C - 50 PSI
D - 65 PSI
E - 80 PSI


Allowable Max Stated Cold Pressures:

Passenger Car (P)
SL - 35, 41, 44, 49, 51 PSI
XL - 41, 44, 49, 51 PSI

Note: Never exceed max stated cold pressure on a Pass Car Tire.

Light Truck (LT)
C - 50 PSI
D - 65 PSI
E - 80 PSI

Note: By TRA specifications on LT rated tires only cold pressure may be increased a maximum of 10 psi over stated max.

 

[CapriRacer]

Gene,

Great post, but many BITOGer's delight in finding exceptions, so I think some additional information may be in order.

Quote:

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Pressure at which maximum capacity is rated:

Passenger Car (P)
SL - 35 PSI
XL - 41 PSI

Note: When used on vehicles classified as trucks passenger car weight ratings must be reduced 9.1%.

Light Truck (LT)
C - 50 PSI
D - 65 PSI
E - 80 PSI

---

First, the passenger car rating pressure is 35 psi for standard load P metric tires (US /TRA standard) just as stated, but a lot of tires are built to European or Japanese standards and for those passeneger car standard load tires, the rating pressure is 2.5 bar (250 kpa) which is about 36 psi.

The LT rating pressure is pretty much as stated, but this isn't true 100% of the time. You will find some LT tires where a Load Range D is 50 psi, Load Range E is 65 psi, etc. The reason for this is fairly complex, but it has to do with the bead to bead length of the interior of the tire - the path the cord takes - so you will see this on tires with larger section widths. The crossover point seems to be in the 315 range for higher aspect ratios, and is wider for lower aspect ratios.

AND just to muddy the waters a bit more!!

It sees some tire manufacturers haven't figured out the part about rating pressures and maxmium pressures.

There is a photo somewhere in this section of BITOG of a Uniroyal that says "Maximum Load XXXX at maximum 35 psi". I'm hoping that everyone understands that there is something wrong with this and that this doesn't follow the "rules".

 

[tigrpal]

Here is that image:
[image] [/image] (Click on it to enlarge. Not sure how to post it larger)

These tires are original on an '03 GMC Sonoma.

CapriRacer, we had this discussion and Gene is right, I'm among the many who are confused, mostly because the door tag says to inflate to 35 psi. Which seems to me that GM is saying that I should run these tires at their max. inflation. That seems out of the ordinary to me.

 

[CBODY67]

What the "door tag" notes and what the tire sidewall notes are two different, but related things.

The door tag used to note variations of tire pressure with "usage", as the owners manual used to note (i.e., smooth ride, minimal passenger load, no "trunk" load . . . or continuous speed above __mph . . . or "full rated load". Each type of operation had their respective recommended tire inflation pressure. The tag also listed the "minimum" tire size spec too, but not accpetable optional or "additional equipment" tire sizes (which caused some confusion in its own right).

IN the more current times, the door tag notes the factory recommended tire pressure for (apparently) the uses the vehicle can now "see". In more modern times, trailer hitch uses and vehicle loadings are not as widely variable as they were 30 years ago, by observation, which makes the tire pressure recommendation a little easier to make. This same pressure recommendation would enhance tire wear with a good balance of performance AND fuel economy, typically.

What's on the tire's sidewall is related ONLY to the tire and not the vehicle it's mounted on. If, for example, the vehicle recommendation is "30psi", but the tire sidewall notes a max pressure of "35psi", the door tag should be what is used, UNLESS there is a reason to use the higher inflation pressure--COLD INFLATION PRESSURE, not "hot" after it's run a while.

Also, consider the door tag as something of a "minimum spec" rather than a "max spec" (which is what's on the tire sidewall).

It seems that with the tire manufacturers and auto manufacturers working a little more closely together these days (especially GM as they have a complete group devoted ONLY to tire specs . . . the "GM TPC _____" spec number that you see on the sidewall of GM's OEM-provided tires). This would mean that provided tire tread widths are more matched to a particular rim width range so they'll wear better at the recommended higher pressure (a tighter range than what the rim width spec might be on a tire company's website spec sheet).

In the days when "32psi" was the max pressure for normal tires, "XL" rated tires (as in 6-ply rated P235/75R-15XL tires that you could order OEM on GM pickups in the later 1970-early 1980 time frame) went up to about 36psi and had a higher load capacity than the provided 4-ply rated P235/75R-15 tires.

I bought a set of Pirelli P77s one time. Their sidewall "mas pressure" rating was 45psi but were "SL". I asked a tire company phone rep about the load rating and why they had the higher pressure spec. He replied that the load rating was the same as at 35psi (for P225/70R-15), but the higher 45psi was there for "racing purposes". I thought that interesting.

In the more current time, though, there can still be some variables. For example, you might find better handling with about 2psi more air in the front than in the rear, to compensate for the more forward weight bias vs. available tire weight-carrying capacity of many vehicles, but not going below or above either the factory recommendation AND the tire sidewall's notations.

Remember that what's on the door tag is for that specific vehicle, yet the tire on the vehicle can be on many other varied vehicles that might NOT use the same manufacturer-recommended inflation pressure as where the tire might be now.

Just some thoughts,
CBODY67

 

[MNgopher]

My '97 and '99 F150 recommended pressures at the max presure on the OEM sidewalls. the '97 (2wd) recommended 32/35 (sidewall max was 35) and the '99 (4wd) recommended 35/35 with a sidewall max of 35 psi.

If that's what the recommended pressure is by the manufacturer, I don't see a problem.

 

[Gene K]

Quote:

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CapriRacer, we had this discussion and Gene is right, I'm among the many who are confused, mostly because the door tag says to inflate to 35 psi. Which seems to me that GM is saying that I should run these tires at their max. inflation. That seems out of the ordinary to me.

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This is not as unusual as you would think. My Ford F150 has P255/65R17 Michelin LTX AS Tires rated at 2205 lb @ 35 psi Cold. Because they are "P" rated tires you have to reduce the rating to 2005 lb. What does Ford suggest for inflation pressure? 35 psi Cold.

This post was inspired by a friend who bought P245/70R17 rated at 2205 lb and 51 psi cold. He has the same truck I have and thought if he reduced the pressure to 35 psi per factory recommendation the tires would not support the weight. I had to explain to him how the tires were rated and that the "overpressure" was allowed for high speeds or increased stability under near maximum loads. However the tires would still be 2205 lb (2005 lb Truck Rating) at 35 psi.