Tire pressures

[JJump]

Hi there TDR fans!



I'm new to the list, being a first time CTDl owner & a first time RV hauler. I posted this thread in the first gen forum & it was recommended I do the same here.



Being new to this whole heavy hauling arena, I could use some advice on tire pressures. My rig is a '93 D350 LE dually extended cab with 235/85 x 16" tires and it pulls a 30' 5th wheel travel trailer. I pulled the combo (truck & trailer) onto a scale at a local truck stop. The scale was configured in 5 sections-kinda like 5 scales set up end-to-end. The total weight (13,700 lbs) was compiled from 3 of the 5 scales; both axles of the truck were on scale 1, and although I didn't take notice, from the data I presume that one trailer axle was on scale 2 & the second trailer axle was on scale 3. Scale 1 showed 8600lbs, scale 2 showed 2500 lbs, and scale 3 showed 2600 lbs. I dropped off the trailer & came back to get an empty weight for the truck, which was 6600 lbs. , all on scale 1.



These numbers surprised me. I never would have guessed that I had 2000 lbs tongue weight-the truck doesn't seem to settle that much with the load. That's also a little more than 28% of the total load, well over the 10-15% that I've heard is the general rule of thumb for tongue weight. The trailer weight was a big surprise too, seeing that both the previous owner & the NADA listing for the trailer stated its weight was 4200 lbs. Since then I've done some research, and actually, 7100 lbs for a 5th wheel this long is kinda on the low side. Makes me wonder if the truck scale was accurate?!



Back to tire pressures. While driving the rig (truck & trailer) back from AL to MO (mostly interstate, 60-65 mph) I was monitoring the tire's temperatures , and they always were just warm to the touch-maybe 85-90 deg F. I recall the tire pressure being around 40 psi. I've read that the way to determine correct tire pressure is to take a reading when cold & then take another when up to operation temp, like after a 15-20 mile jaunt at speed. The correct cold pressure is arrived at when you achieve a 10% increase from cold to operating pressure. Sounds like a good plan, but I always forget to take the hot pressure- Doh!



I would appreciate any input. Hopefully it might influence my fuel economy & extend my tire life. Thanks for your time!



Joe Jump

St Louis

 

[barkerdl]

You did not say what brand or rating tire you are using. I run Michlens load range E same size as your 93 on my 92 pulling a 6500# fifth-wheel. Gross combo weight is 12000#. I ran 60 in the front and 70 in the rear with good tire wear and no tire problems. Running Goodyear ST 225R75x15 load range D at 50 psi on the trailer. Just my opinion on the Michlens, but Goodyear has a web site with tire inflation for their tires.



Dennis

 

[pwr2tow]

My 33' Jayco camper weighs in at 12,000 lbs pulled across the scale and a total of 20,000 lbs combined with fire wood in the truck bed, truck weighs 7500 lbs.



I don't know that much about useing cold and hot inflation numbers to come up with the correct tire inflation. I just read the max inflation numbers on the tires for the trailer and run them at that. I have pulled campers for 20 years and have never had any tire problems, zero flats or blow outs. On the truck I run 70 psi front and 60 psi rear on over sized (for my truck) 235/85/16 tires, stock is 215/85/16.



Ron

 

[Vic]

Originally posted by JJump

Hi there TDR fans!



"Back to tire pressures. While driving the rig (truck & trailer) back from AL to MO (mostly interstate, 60-65 mph) I was monitoring the tire's temperatures , and they always were just warm to the touch-maybe 85-90 deg F. I recall the tire pressure being around 40 psi. "



Hello Joe



40 psi sounds way too low. Whatever your tire size, would expect you should be running either 50 psi or 65 psi, but probably 50 psi because if you had 65 psi tires, running it at 40 psi would be clearly way too low.



I've pulled a light Terry fiver for over 100,000 miles with never a tire problem. Running 50 psi, max per sidewall. Then got a heavier fiver, used, and had three tire failures within a year. Got all new tires, and carefully run them at 65 psi (max per sidewall) and no tire problems since.



Didn't know about those segmented truck scales. Will look for one when we shortly head for New Mexico and a winter wandering around in the South West.



Good luck, Vic

 

[Casey Balvert]

The tongue weight that you quoted of 10 to 15 % is for a tag along. Fivers generally run 20 to 25% on the pin so you are pretty close.



Weigh the individual axles on your truck and check the Goodyear site for inflation pressures for various loading for your tire size to be sure you are not running underinflated. As far as trailer tire, I run them at the max cold pressure indicated on the sidewall. Trailer tire are usually sized to be JUST adequate.



FWIW



Casey

 

[JJump]

Thanks so much for the insitghtful replies!



To answer a couple questions..... This morning I wrote down some info from the tire sidewalls & checked pressures. I have 2 different brand tires, 2 Michelin & 4 Cooper. All are load range E and list a max cold pressure at 80 psi. I was mistaken in the pressures-the tires were at 55-60 psi (except for one that was at 40). I'll go to the Goodyear site as recommended & see what I can find there. ALso worth noting is the sticker placed in the driver's door jam lists 50 psi, but that may apply only to OEM tires, and judging from another response, my tires are oversize at 235/85 x 16".



Glad to hear that the 10-15% rule doesn't apply to 5th wheels. Makes me believe that the scale may have been somewhat accurate.





Thanks again for the help-appreciate your time!



Joe Jump

St Louis

 

[cwsoules]

The label on the drivers side door pannel calls for 50 psi forward and 70 aft. I've done that for nearly four years with good results. I tow a 10K 5er and have put 30,000 miles on it while putting a total of 60,000 miles on my tires before replacing them.