question about tire pressure

[landis_rogers]

I am confused about how to set the tire pressure on my truck. I looked at the door pillar sticker and it says 70 front & 60 rear for bvw of 13,000 lbs. The tires say 80 psi for max inflation pressure. The tires are the stock tires. How do I know what to inflate the tires to. Do I adjust the pressures according to the load? Any suggestions would be great.

[haul_n_horses2]

You will get the best mileage out of your tires and the most comfortable ride in your truck if you inflate the tires based on the load. Take your truck to the CAT scales nearest you and weigh it with a full tank of fuel and the typical passenger/tool load that you carry. It will tell you the weight on each axle. Then go to the tire and rim manufacturer inflation tables found online and look up the psi necessary for the weight on each axle. Take the weight on the axle and divide by two (4 if drw) and that is the weight per tire. The table will tell you what psi to inflate to carry that weight. I run 45 psi rear/ 55 psi front when empty and go up to 60 psi rear/65 psi front when I am hauling the heavy horse trailer. Another method is shown here.

[SmokeyWren]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: landis</div><div class="ubbcode-body"> Do I adjust the pressures according to the load? </div></div>

Yes.

The PSI on the Federal Certification Label is only for when you're loaded to the gills. The 80 PSI on the sidewall of the tire only tells you that you have load range E tires with a weight capacity of 3,640 pounds if you have enough truck to haul that much weight. (You don't.)

Use a load/inflation table for your tire size. That means you have to know the weight on the front and rear axles. So get one weight when loaded for bear, and another weight for when "unloaded" but with your normal stuff and driver and passengers in the truck.

If your truck is a 4x4 SRW, then you probably have LT275/70R18E tires. Here's the load/inflation table for that size:

PSI = max weight per tire
35 = 2,070
40 = 2,270
45 = 2,470
50 = 2,680
55 = 2,840
60 = 3,020
65 = 3,195
70 = 3,360
75 = 3,530
80 = 3,640

So if when "unloaded" your truck weighs 3,300 on the rear axle and 4,500 on the front axle for 7,800 GVW, that's 1,650 pounds on each rear tire and 2,250 on each front tire. In that case, you need 35 PSI in the rear tires and 40 PSI in the front tires.

When loaded for bear, if the rear axle weighs 6,500 pounds and the front axle weighs 5,000 pounds for a GVW of 11,500, then you need 70 in the rear tires and 50 in the front tires.

In other words, your tires can carry a lot more weight than your truck can safely haul without exceeding the 11,500 pounds GVWR of a CrewCab 4x4 longbed PSD.

[Cool_Canuck]

SmokeyWren will be all over this. [img]/forums/images/%%GRAEMLIN_URL%%/smile.gif[/img] He is one who understands proper tire pressures better than most on this forum. (see, I told Ya)

The door sticker IRCC is the tire pressure to meet the Gross Axle Weight Rating. A weight, that if max on both axles would exceed your Gross Vehicle Weight Rating. Very stiff ride if empty.

The PSI on the tire side wall is of course the maximum pressure you can inflate the tire to while cold.

You really need to set tire pressure to the weight carried and the speed traveled. I know the weights of my steer axle and drive axle while empty and loaded with my 5th wheel. I set my tire pressures accordingly. As an example, with my 5th wheel loaded both the steer and drive axles weigh close to 4,400 lbs. 55 psi both front and back is ample. I do run them closer to 65 just to cover a margin of error.

The tire manufactures put out inflation charts but they are getting harder to find. Actually it is a tire association that puts out the charts so for a given size the numbers are all the same for all the manufactures. They will generally leave out sizes they do not make. [img]/forums/images/%%GRAEMLIN_URL%%/smile.gif[/img]

As for the speed angle, light truck tires tables are for speeds up to 65 mph. My GoodYear table indicates that for speeds between 66mph and 75 to increase tire pressure by 10psi. For speeds between 76mph and 85 mph increase the tire pressure by 10 psi and REDUCE the load by 10%. In no case should the tire pressure indicated in the table be increased by more than 10psi.

There are also tables for reducing speed and increasing weight.

Over inflated tires wear faster, have less traction and ride hard.
Under inflated tires wear faster, have less traction and generate heat which in turn cases tire failure.

Here is an online chart from Michelin. The is a down loadable .pdf at the bottom.

[toalpi]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Cool_Canuck</div><div class="ubbcode-body">As for the speed angle, light truck tires tables are for speeds up to 65 mph. My GoodYear table indicates that for speeds between 66mph and 75 to increase tire pressure by 10psi. For speeds between 76mph and 85 mph increase the tire pressure by 10 psi and REDUCE the load by 10%. In no case should the tire pressure indicated in the table be increased by more than 10psi.


</div></div> If you read the fine print you will see that this is for tires that are speed rated for 65mph. Your modern LT tires are rated for much higher speeds. A few decades ago you would have been able to find LT tires with 65mph ratings. You can still find commercial tires with 65 and 75mph ratings.

[Cool_Canuck]

The fine print
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">INFLATION LIMITATIONS
Light Truck Tires
The inflations shown in the load tables are minimum cold pressures for the various loads listed. Higher pressures should be used as follows:
A. When required by the speed/load table, table 1 on Page L2.
B. When higher pressures are desirable to obtain improved operating performance.
The combined increases of A and B should not exceed 10 psi above the inflation specified for the maximum load of the tire.
THE MAXIMUM RIM CAPACITY MUST NOT BE EXCEEDED.
LOAD LIMITS AT VARIOUS SPEEDS FOR DIAGONAL (BIAS)
AND RADIAL PLY TRUCK TIRES USED ON IMPROVED SURFACES
(These tables do not apply to rims or wheels)
The Tire and Rim Association permits tire load increases, often with increased inflation pressure, for both Truck-Bus tires and Light Truck tires used on improved surfaces at
reduced operating speeds. In addition, the Tire and Rim Association also permits operating a 65 mph-rated tire at higher speeds with a reduced load and increased inflation.
(The Goodyear Tire & Rubber Company does not condone or recommend operating speeds above posted limits.) Goodyear accepts these increases, and they are published in
our truck tire engineering data book. Rim and wheel manufacturers mark their products with a maximum load and inflation. This applies regardless of operating speed. The
rim/wheel manufacturer must be contacted to determine if any deviation is permitted in the marked maximum load and inflation capacity of the rim or wheel at the operating
condition in question. For further details and a worksheet covering the use of these tables see pages L-4 and L-5.

TABLE 1 Ñ LIGHT TRUCK TIRES
For LT Tire Sizes Only. (i.e. LT235/85R16, LT245/75R16). For 225/70R19.5, 245/70R19.5, use table 2 below.
The service load and minimum (cold) inflation must comply with the following limitations:
SPEED RANGE INFLATION PRESSURE %INCREASE (+) OR
(MPH) INCREASE DECREASE (-) IN LOADS
76 thru 85 + 10 PSI - 10%
66 thru 75 + 10 PSI None
56 thru 65 No Increase None
46 thru 55 No Increase + 9%
36 thru 45 No Increase + 16%
26 thru 35 No Increase + 24%
16 thru 25 No Increase + 32%
11 thru 141) + 10 PSI + 50%
6 thru 101) + 10 PSI + 65%
1 thru 51) + 10 PSI + 80%
Stationary1) + 20 PSI + 165%
1) Apply load increase to dual loads only (even if tire is in single application). Source: Tire and Rim Association</div></div>
Sorry, It didn't cut and paste real well, however the information is there. This information is not that old and I see nothing about speed ratings on LT tires. I know it is on P rated tires. How would I know the speed rating on my LT tires? I see nothing that would indicate a speed rating on my Michelin LTs.

Here some interesting facts on tire service. At the bottom of the page is a link to speed ratings on tires.

[SmokeyWren]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Cool_Canuck</div><div class="ubbcode-body"> How would I know the speed rating on my LT tires? I see nothing that would indicate a speed rating on my Michelin LTs.</div></div>

Assuming LTX AT/2 in size LT235/85R16E, the speed rating is "R". I found that by looking at the specs on the MichelinMan website. Most LT tires are speed rated Q or R, with some rated M, N, or P.

On that TireRack website you linked to, you can find the translation into English of what the speed rating means. Here is the most common speed ratings for LT tires:

M = 81 MPH max
N = 87
P = 93
Q = 99
R = 106
S = 112
T = 118

[Cool_Canuck]

Hummmm. My Michelin MS 265/75R16/E hurtling down the road @ 106 mph with 3,830 on each corner. Scary! I see now where all LT are speed rated R and LT winter tires are Q. The day was not a waste. [img]/forums/images/%%GRAEMLIN_URL%%/smile.gif[/img]