Tire Pressure Question

[Archive]

Well, I finally got around to weighing my truck at work, and here are the specs:
Front = 4060 lbs
Rear = 2840 lbs

Now, my tires are 285/75r16s which are rated for 3305 lbs at 65 PSI max. When I run the forumla, I get the following results:

Front
(4060 lbs / 2 )/(3305 lbs) * 65 PSI = 40 PSI

Rear
(2840 lbs / 2 )/(3305 lbs) * 65 PSI = 28 PSI

40 PSI for the front sounds OK, but does the 28 PSI sound a little low? I'm currently running 40 PSI all around. Probably a little too much for the rear.


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Scott Collins
'98 Ram 2500 4x4 12 Valve Cummins.
Renton, WA

 

[Archive]

I too ran the formula and did not like what I got. My tires are rated at 3415 @ 80psi. To be running at 80 psi I would need to be grossing 13,660 on the truck alone. Since this would be a severe overload on a truck rated at 8800 GVWR, I wonder if this formulae is valid. It makes sense, but even at maximum GVWR of 8800 pounds, the tires will not be inflated anywhere near the maximum pressure. On my truck, I am running about 8150 all of the time, with 4500 on the front and 3650 on the rear. The formula gave me 53 psi front and 43 psi rear. I currently have 60 psi all the way around. Any advice? I have some steering wander and am wandering (pun intended) if my tire pressure has something to do with it?

Happy Thanksgiving

Shelby

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2001 2500 QC LB 4WD, ETH, DEE, 3. 54 gears, Laramie SLT, Camper Special, Trailer Tow, Travel Convenience Group, Power Driver Seat, Alarm System, Remote Keyless Entry, Fog Lamps, Cab Clearance Lamps, HD Transfer Case, Bright White with a Gray Interior. Built 09-15-00, took delivery on 10-06-00. Custom aluminum work body / flat bed.
Coming soon: E-Brake and gauges.

[This message has been edited by Surveyor Shelby (edited 11-23-2000). ]

 

[Archive]

Shelby

A
First I am no tire expert. All that you say is true. The weight carrying capacity of your tires is greater than the Total of your truck. A nice margin to have. You could run smaller tires at a higher pressure or what you have at a lessor pressure for a more comfortable ride. I run 60 front 40 rear. Close enough for my formula

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Barry Horner
Lake Forest
Orange County, Ca

2001, SLT+, ETH, 6 speed, White quad cab, short bed, Sport, 2wd , 3:54 anti spin, Camper Special with trailer tow, 16x8 Forged Aluminum with 265x75 tires, Fog lights, Audio controls steering wheel mounted, AM/FM cassette with CD, Travel Convenience group, Keyless entry with remote, Leather agate heated seats. Line-X bed liner. ARE LS 2 tonneau cover. Pillar mount boost and EGT. EDGE EZ and soon to come McLeod clutch.

 

[Archive]

Scott

Try using 80 psi. Most of the Dodge trucks show that as a max with supplied tires. Check your door post.

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Barry Horner
Lake Forest
Orange County, Ca

2001, SLT+, ETH, 6 speed, White quad cab, short bed, Sport, 2wd , 3:54 anti spin, Camper Special with trailer tow, 16x8 Forged Aluminum with 265x75 tires, Fog lights, Audio controls steering wheel mounted, AM/FM cassette with CD, Travel Convenience group, Keyless entry with remote, Leather agate heated seats. Line-X bed liner. ARE LS 2 tonneau cover. Pillar mount boost and EGT. EDGE EZ and soon to come McLeod clutch.

 

[Archive]

My truck stays loaded with a full cord of split oak. (work truck) I have been running 80psi. all around and have over 30,000 with still plenty of tread left. Cant see any since changing. Rides and handles well. OH I run the stock Michelins.

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99 Dodge 4x4 Quad Diesel auto. K&N, 4" JRE, 5" chrome tip, matched 275 Bosch injectors, 2. 5" Skyjacker leveling kit, Silencer ring MIA. (more to come)

 

[Archive]

Originally posted by BHORNERIS:
Scott

Try using 80 psi. Most of the Dodge trucks show that as a max with supplied tires. Check your door post.

I don't have stock tires anymore, so I don't think the door plaque is valid any longer. The stock tires were 245/75r16s and now I have 285/75r16s. I checked the sidewalls on the tire, and they specify a max pressure of 65 psi cold. So, I don't really think 80 psi would be advisable... ... .


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Scott Collins
'98 Ram 2500 4x4 12 Valve Cummins.
Renton, WA

 

[Archive]

Scott

I hope I did not mislead you in thinking that you needed to run 80lbs. In order to run 80 you would really have to load it well above capacity. Just use it as a constant to figure tire pressure for best traction. If they are over inflated or under inflated you do not have the optimum tread foot print. You
can over inflate and rotate often and still get good mileage but not the best contact patch. Underinflation is the real killer.

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Barry Horner
Lake Forest
Orange County, Ca

2001, SLT+, ETH, 6 speed, White quad cab, short bed, Sport, 2wd , 3:54 anti spin, Camper Special with trailer tow, 16x8 Forged Aluminum with 265x75 tires, Fog lights, Audio controls steering wheel mounted, AM/FM cassette with CD, Travel Convenience group, Keyless entry with remote, Leather agate heated seats. Line-X bed liner. ARE LS 2 tonneau cover. Pillar mount boost and EGT. EDGE EZ and soon to come McLeod clutch.

 

[Archive]

Oh, now I understand what you meant. Use "80" for the calculations, but not necessarily run 80 psi. So, if I run the forumula using "80" I get the following:

Front = 49 PSI
Rear = 34 PSI

Those sound a little better than previous the numbers, but I'm still not sure. I even searched the entire web for tire pressure info, and every site says the same thing "check with the tire manufacturer or the vehicle's owner manual. " So I check with the tire manufacturers, zip, zero, zilch. What a joke.

I for one do not believe that a tire's load capacity is linear with respect to tire pressure. I would expect it to be an exponential, increasing curve, where load capacity is not signicantly increased until you get to the upper end of the PSI range. Of course, that would mean that every tire would have a different curve, depending on tire construction. Calculating this curve would be quite tricky.....

The end result of all this? I think I'll put a few more PSI in the front, and leave the rear alone, and watch for tread wear patterns... ...

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Scott Collins
'98 Ram 2500 4x4 12 Valve Cummins.
Renton, WA

 

[Archive]

Shelby:
I had wander problems, too. It didn't go away completely even after having the track bar and steering box replaced. Then I checked the tires. They were about 60 all the way around. I bled them down to 45 rear and 50 front (I think), per the tire inflation chart that came with my owner's manual. Now the wander is gone, and it steers like it always should have.

By the way, the track bar and steering box were definately defective, and the tires were properly inflated when I first had the truck. I figure the dealer must have pumped them up one of the times when it was in for the steering problems. Just my experience.

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2001 2500; SLT; 4x4; QC; LWB; Auto; 3. 54 LS; 265s; Camper; Tow; Sliding Rear Window; Travel Convenience Group; Cab Clearance Lamps; Forest Green

 

[Archive]

Dave,

Thanks, I do remember the posts about your problems. I have the AF box and only have about 1500 miles on the truck, so I am assuming that I should not have anything wore out yet. Will try playing with the tire pressures.

Shelby

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2001 2500 QC LB 4WD, ETH, DEE, 3. 54 gears, Laramie SLT, Camper Special, Trailer Tow, Travel Convenience Group, Power Driver Seat, Alarm System, Remote Keyless Entry, Fog Lamps, Cab Clearance Lamps, HD Transfer Case, Bright White with a Gray Interior. Built 09-15-00, took delivery on 10-06-00. Custom aluminum work body / flat bed.
Coming soon: E-Brake and gauges.