Tire pressure formula -surprising results

[mbrolin]

Hooked up the new 5er today and headed to the scales. Front axle - 4550lbs Rear axle - 5300lbs 5er axles - 9000lbs. Using a formula posted a few months ago yielded tire pressures of Front - 55 psi Rear - 65 psi 5er - 70 psi. These numbers have been rounded to the higher pressure ie front actual is 53. 2 lbs. Makes sense since the maximum tire weight on truck is 3415 at 80 psi per tire. BTW,the formula was axle weight divided by # tires divided by tire rating x max psi = tire pressure.

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2001 HO 6spd,Quad cab,LWB,4x4,Flame Red,all options except for leather,Westin Brushguard/Push bar,Painless wiring,Radio Shack CB,my own design CB mount,Smittybuilt nerf bars,NW Custom rocker covers,Custom built stainless rocker light bars,DeeZee bedrails,Isspro pyro and boost gauges,Jacobs ebrake,Reese 20k 5er hitch(more to come)

 

[Okie-Go]

I thought that formula seemed a little low. I run 60 on all corners. Loaded I put the rear up to 75 and the 5er to 75. Max is 80 on all of them. They are doin ok.

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2001 quadcab slt 2500 HO 6spd. LWB,2wd,dk garnett red, trailer tow package,camper special, lock-right locker, 3. 54 gears, speed liner,oversized stainless steel chicken slide,66 gallon in bed aux. tank,K&N air filter,Reese 20k hitch, Terry 2000 EX 30ft. double slide 5er, 2000 20ft gooseneck for haulin jeep, firewood and huntin stuff. 5X12 tagalong for haulin the 4 wheeler. Okie Newton

 

[PUFF00]

Michael,

Your post raises the question of what “load rating” actually means. My intuition suggests to me that the formula you state is inadequate. Two parameters it doesn’t include are vehicle speed (tire revolutions/min) and tire temperature. It also suggests that the load rating is linear with tire pressure, i. e. , divide pressure by 2 and the load rating of the tire at that pressure is max-load-rating/2—perhaps this is true, but I don’t know that and would not assume it off-hand.

Just focusing on the vehicle speed: there is a difference between how the tire will hold-up at a given pressure at different speeds. If you are driving at 80 mph for a sustained time you need a higher tire pressure than if you are driving at 40 mph. One reason for this is heat build-up—lower pressure = more tire flex = higher temp at any given speed.

I would take the formula you are using with a large grain of salt. I don’t have any data to offer, so I’ll just offer my intuition: if you are driving on the highway at high speed (70-85 mph) then air-up to max or near max pressure regardless of your load.


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2001. 5 3500 QC 4X4 6-speed HO 3. 55 black
1998 Jeep TJ black http://www.nm4w.org/members/puffer.htm

 

[claymike]

I run 60lbs in the front and 70 in the rear. Always leave the 5er at max. Have towed many thousand miles and never had a problem.

Mike

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Mike Clay
2001 HO,6 speed QC,2500,4x4Dark Garnet Red,ISSPRO E&B gauges on A Pillar, Rhino Nerf Steps Lit, Series 3000 brush & grill guard, NW Custom rocker panels, Jacobs E-brake, all options except leather, Reese 20k hitch. Towing a 30' Gulf Stream Sea Hawk 5th Wheel triple slide. The Mule

 

[Guest00]

I'll agree with Puff.
I have never lost a tire that was inflated to max but have lost several that were not maxed reguardless of the load. M/T I run 65/f and 55 rear. I would rather prematurely wear the center of the tread than the outer sides of the tread. Attaining the PERFECT pressures is a trial and error process, the problem is the error part, as the seasons, loads and roads change so does the PERFECT pressures.
I err on the side of safety and usually run over inflated in reguards to the PERFECT pressure. Since my rig is not maxed out on axle/ tire weight I usually inflate 5 psi under max. In 30 yrs of towing. I have never lost a tire cept to unavoidable road hazards, never a blow out, tread seperation or side wall failure. ( knock on wood ). Underinflation is the #1 cause of tire failure.

JMHO
Jack