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Re: Can someone explain the dogma against P tires ? [message #371034 is a reply to message #371025] Tue, 31 January 2023 09:46 Go to previous messageGo to previous message
Matt Colie is currently offline  Matt Colie   United States
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
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One of the very few inconveniences of being retired from the halls of Detroit is that I have lost contact with so many good sources of the factual information. This particularly true as much of my time in the OEs was warranty verification. (You might be surprised at what some people think should be covered by the warranty.) I was kind of hoping someone with more current experience might join in.

In an extended discussion with my Firestone friend (a now lost contact) when I was still an inexperienced coach owner, he suggested that our coach could well be fit with a P-metric tire except that I had not yet changed out the rims to 16. A lot of the discussion was actually about the future availability of 16.5 tires. (My real reason for the contact.) As it happens, they had recently taken one set of tooling out of line at one plant, but were going to keep up production at a lower rate for a very long time because that tire (specifically the 8.75) still had significant demand in the agricultural sector.

As usual, our conversation covered a lot of ground. He was aware of but not exceedingly familiar with GMCs and stated that there had been problems in multi-tire axles with P tires. In case of a dual (like a P30), not all dual mounts can accommodate the side wall shape of a P tire at load. P tires do have softer side walls, and in a dual configuration, the sidewall can contact each other and apparently this is a very bad situation. His description of the difficulties was extensive, but I really didn't retain it because it was not a case that interested me. He did say that tandems (what we have) also do not fair real well either as the scrubbing on short radius turns did could cause the tread belts to try to slip. This should not be an issue with a first quality tire.

There was an abrupt halt in the discussion at that point as he apparently accessed another mental file and had to shift gears to get things aligned. Side Walls? I had told him I did not have duals, but instead had a tandem rear axle. This was the speed bump in the discussion. He asked very specifically if we hit curbs with the rear tires. I allowed as this only happened when we turned corners. (This was actually not accurate but more on this later.) The actual issue here was that the LT tires do have heavier and stiffer side walls and are designed to tolerate a "curb hit" (he had an inside term for this) and the P tires are not there at all. This is why the ride is much softer and the lateral slip is less. I personally have never drifted the coach and do not intend to, but I cannot make this claim for my passcars.

As to rolling resistance, yes, a P tire of same size at same load will be lower than the LT. He was not specific as to how much lower. He allowed that an OE had recently been accused of doing the tests for CAFE for light trucks with P tires to get better numbers. I did not collect specifics, but I do believe that the case could exist. While at Jeep/Truck in the late 90s, we were told by the EPA that our recommended lubricants were "too low a friction" to be accepted for standard tests. In durability, we didn't change anything, but I know that the certification people did.

At the tail of all this, he injected something about the ag tires having better UV resistance and other things to improve the calendar life. I could not grasp how much effect this might really have.

That is pretty much what I know about the subject. So, I will end the technical part of this here. Maybe we can get Roger Marble to an international some year. He has been to several FMCA events.

I have also had great success with the two at a time rotation plan. When I discovered how much trouble I was really in, I bought a full set for the coach and that was a big bite to maintenance budget. I am glad I am not throwing away 22.5s. If you look at my coach pictures, you can notice that some times there is a mix of wheel colors. The white ones are 16 and those in gray prime are the 16.5s.

Oh, the side wall non-turning issue was that I used to regularly take Chaumière over a steel grate bridge that had high steel curbs and 9' wide lanes. If I was not real careful, those right side read rims could hit the high steel curb. I don't do that trip anymore with the powder coated white rims.

A specific aside for Dave (the OP), if you are still planning a bonsai run, P tires might get you a little more tank range and thus get a little longer between stops.

Matt

Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

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