Interestingly, the correct answer appears to be "None of the above".
Tire manufacturers for the US market use essentially the same inflation pressures for the same tire size with some slight variations. The load range does not enter into it at all except that higher LR tires can have more pressure in them. The lower pressures are the same regardless of the LR. The wheel width does not enter into it at all except that the wheel must be able to withstand the expected pressures.
STICKER REMINDER:
Ford: 95 PSI front and 100 PSI rear
Winnebago: 80 PSI front and 80 PSI rear, which is actually dangerous on the front.
Under FMVSS 120, which covers vehicles with a GVWR in excess of 10,000 lbs, the sticker must show the
minimum pressure needed in the tires installed by the final vehicle manufacturer in order for those tires to safely support the maximum design weight of that axle, its GAWR.
The front axle is 7,000 lbs GAWR or 3,500 lbs per tire.
95 PSI is the correct sticker pressure for the front axle. That pressure supports 3,640 lbs per tire * 2 or 7,280 lbs, or 280 lbs over GAWR.
80 PSI would only support 3,195 lbs per tire or an axle GAWR of 6,390 lbs, 610 lbs below the actual GAWR.
The rear axle is 12,000 lbs GAWR or 3,000 lbs per tire.
80 PSI is the correct sticker tire pressure for each tire on the rear axle. That pressure supports 3,000 lbs per tire * 4 or 12,000 lbs, or exactly the GAWR.
100 PSI in the rear tires would support 3,490 per tire or an axle GAWR of 13,960 lbs with Goodyear tires. That's 14% above GAWR.
If I did all of the math correctly, of course. Please double-check my numbers.
Winnebago would not be the first motorhome manufacturer to have a recall because the sticker was wrong but there was no recall.
Quote:
Originally Posted by Gary RVRoamer
When selecting the tire psi from the inflation table, you must make an allowance for both future weight change and future "cold" temperature change. Unless, of course, you are willing to check the psi at the beginning of every travel day and make suitable adjustments. Most owners add some extra psi to accommodate variations in both. 5 psi is a common "fudge factor".
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Gary is correct but there is another factor. It's not just the expected temperature variations during your travels it's the problem that many RV's and motorhomes are lopsided. When a four-corner weight is done the owner can see just how lopsided theirs is.
Why is this a problem? Because the tire load inflation charts assume a perfect left-right weight distribution and it's well-known that a left-right weight imbalance up to 10% on an RV is not that uncommon. And an axle weigh on a CAT Scale divided by the number of tires on that axle only shows the
average load on each tire.
As an aside, the one pic is of a friend's Grand Design 303 RLS fifth wheel. He got a four corner weight (actually a 5 corner) and that was the result. His pin weight was 23.5%, which is about perfect, but do the math on those axles left to right. His is completely stock with zero mods.
When using an axle weigh, a CAT Scale, I add 5% to each axle weight and only then do I look up the pressure in the load inflation charts. That "fudge factors" for the probable weight imbalance. I then add another 5 to 10 PSI to that figure for temperature changes.
If using just the
correct sticker pressure, I add 10% to those numbers and it works about perfect for us.
Our minimum, and the sticker pressures, are 90 PSI. When leaving southern Florida in early April I make sure the tires have about 100 PSI in them at 80 F. When arriving back in Ohio a week later it's in the 30's and the tires are just above 90 PSI. Throughout the summer the pressure increases until we're ready to leave for Florida again.
If I had the OP's motorhome I would be running 100 PSI in the fronts and 90 PSI in the rears
if neither axle is overloaded.
100 PSI in the fronts would support 7,430 lbs or 6% above GAWR.
90 PSI in the rears would support 12,980 lbs or 7.5% above GAWR.
FWIW,
Ray
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