Discount Tire says, more than once on that web page,
"Always inflate trailer tires to the maximum inflation indicated on the sidewall."
But, for the finicky and careful, Goodyear says that you should "tune" your PSI to the load on the tire. Max PSI is really only correct when the tire loading is at the tire's maximum. Here's the table:
http://www.goodyear.com/rv/pdf/rv_inflation.pdf
And here are the recommendations for the Marathon ST225/75R15, which is the tire most likely to be overinflated by TM owners:
max load <> recommended
per tire: <> cold PSI:
1060 lbs <> 15 PSI
1260 lbs <> 20 PSI
1430 lbs <> 25 PSI
1600 lbs <> 30 PSI
1760 lbs <> 35 PSI
1880 lbs <> 40 PSI
2020 lbs <> 45 PSI
2150 lbs <> 50 PSI (max load and PSI for class 'C' load rated)
2270 lbs <> 55 PSI
2380 lbs <> 60 PSI
2540 lbs <> 65 PSI (max load and PSI for class 'D' load rated)
Now... let's think about a 3023 (the most extreme example, because it has the biggest payload). Dry weight is only 2915 lbs, and TM indicates a load capacity of 1928 lbs. Under maximum load, the axle is at about 4770 lbs (with 13% of the payload moved forwards onto the tongue, but 4% of that pushed back to the TM axle by a WDH). That's less than 2400 lbs per tire, even when the TM is loaded to the absolute max.
Unloaded (less loaded to begin with, coming back home, water tank empty, gray and black water dumped, etc.) you're probably around 3600-3800 lbs on the axle (remember, TM doesn't count options such as AC in dry weight, that's still there). Maybe 1900 lbs per tire, maybe even less.
But, if you were to follow Discount Tire's advice, you'd crank up the pressure on your load range 'D' Marathons to 65 lbs
. Since you're carrying a load of
way less than 2540 per tire, they're not performing with the sidewall flex and tread surface "give" which they're designed to run at. Instead, your running with
rock-hard tires, shaking the heck out of your TM.
Even more important, your tire contact patch with the road is all messed up. Being badly overinflated, the tread naturally tends to bulge out and NOT properly flatten against the road... your "control patch" is just a tiny bit of tread, neither as wide (from side-to-side) nor as long (in the direction of vehicle travel) as it's supposed to be. Instead of a proper balance between pressure and contact size, you've got way to much pressure on way too small a contact patch. That's bad, especially if you're driving in the rain.
- - - -
But even over-inflation at max PSI, with its compromises in handling and suspension properties, is
far less bad than running grossly under-inflated. And, that's what lazy USA haulers are MUCH more likely to do... run too low, not too high. (That was a key factor in most of the Firestone SUV and Truck tire failures, running underinflated.) So, if you want an
easy rule for the lazy and stupid, then Discount Tire says the right thing.
But, it's really not best for the tire, or for safety, or for your TM. If you're finicky and careful, tuning PSI to Loading is definitely better.
Still, if there's any chance that you're gonna let it go low, pump it up as Discount Tire says! You
do not want to drive under-inflated!
ST tires, good information, easy read
Although, maybe *CARLISE* tires can't flex
(Metal fatigue, screws working loose, etc.) And since TMs are famous for good sway characteristics, that Carlisle 'sway advantage' of running a tire puffed up so that it doesn't have ANY side flex isn't needed.
).
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