Computing Actual TP

[five]

I've done the leg work for this question...gotten weights, compared curb side weights with street side weight, etc. I have rounded up to the next weight increment on the table showing the recommended TPs. My question is how much of a fudge factor should be added to the charted PSI for the rounded up weight? The closest I've gotten for an actual suggestion was to add 10 PSI to the chart weight.

FYI, the front two tires are Michelin 315/80R22.5 XZA1s and the rear six are Michelin 295/80R22.5 XZA2.

Thanks.

[CarlAda]

I added 10% to what was on the charts for my weight. I don't know if there's a definitive answer, but better to err on the plus side rather than the minimum pressure for the carried loads.

[bwjoyner]

I use a Tire Monitor system and have noticed that the pressure will increase by 8-10 psi after the tires are warm. This may be something to consider when determining the cold psi.

[wildebill308]

Let me give you a definitive answer.

Lets look at facts tires gain tier pressure when they warm up. Tires are designed to handle this even at the max inflation on the sidewalls. The tire guy can probably give a better idea what the safety margin is but I bet it is fairly good one.

Remember this is the minimum pressure for your load. I bet there is a small cushion built into that table to.

We have people who can't move till they have their tire pressure set to the 0.01 pound and on the other end like the nice gentleman who was walking by when I was checking my tires this summer and asked "why are you doing that? I have the dealer check my tire pressure when they do my annual maintenance." He seamed un-fazed when I mentioned that tires can loose air over time.

Now that is my definitive answer.

Bill

[tireman9]

I've done the leg work for this question...gotten weights, compared curb side weights with street side weight, etc. I have rounded up to the next weight increment on the table showing the recommended TPs. My question is how much of a fudge factor should be added to the charted PSI for the rounded up weight? The closest I've gotten for an actual suggestion was to add 10 PSI to the chart weight.

FYI, the front two tires are Michelin 315/80R22.5 XZA1s and the rear six are Michelin 295/80R22.5 XZA2.

Thanks.

Sounds like you have done due diligence.

If I had your coach I would do the following:

Consult the Load/Infl tables and using the heavier end of an axle establish the MINIMUM Cold Inflation Pressure for all tires on that axle or MCIP

Do the same for each axle

I would then add a margin of at least 10% to that minimum inflation to establish my Goal Cold Inflation Pressure or GCIP

I would set my TPM low air warning level to the MCIP and when checking or setting tire pressure with air hose and hand gauge shoot for the GCIP

Now having said the above, I would suggest you read my blog post of Oct 7 2015 "Don't get your shorts in a bunch about tire inflation"

The +10 psi figure is reasonable when we talk about 22.5 size tires that have inflation levels near 100psi. I am trying to shy away from a specific number of psi as we have some readers that have tires that only need about 50 psi to carry their load. Tire pressure response is really a percent of the base inflation so since we could be discussing anything from 30 psi to 130 psi giving a specific figure can lead to misunderstanding.

About a year ago I ended up pointing out a problem on the Tire Rack web site when they were suggesting tire pressure increases by 1 psi for each rise in tire temperature of 10°F. That figure is essentially correct as the average customer that Tire Rack is talking to is concerned about their passenger car tires but some Class-A RV owners were incorrectly using the 1 psi figure. When I pointed this out to Tire Rack they changed their web page to the 2 percent figure.

and also don't get too concerned about the hot tire pressure. The real change in tire pressure for a temperature increase of 10°F is about 2% as I covered in the technical post "Why does my tire lose pressure? Simple and Technical answers"

RE Safety Margin. will be writing a blog post on this topic in a few weeks as it is a very involved topic. But the short answer is to suggest that you not try and play games with your safety and just assume there is no extra margin or that the margin has been consumed by the damage done when you hit that last pot hole.

As always you should not select a GCIP that exceeds the max pressure for your wheels.

[five]

Good information, Tireman 9, thanks for posting. I have copied your post and added it to my keepers.