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