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Tire pressure question


Z3Driver

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We're picking up our new (to us) motorhome on Saturday -- a 1999 Beaver Contessa DP. First on my list of add-ons is a TPMS and I have a TST 507RV unit set to arrive on Friday.

 

There's a youtube video showing setup. One step references the tires' running pressure. I think cold tire pressure setting is pretty easy to obtain. Is there a generally accepted ratio between cold and running pressures?

 

Thanks,

 

 

Larry

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Tire pressure must always be measured "cold"= when tires have not been operated for at least 4 hours, or less than 1 mile afte beginning driving. The rule of thumb for pressure change is- for every 10 deg temperature change, tire pressure will change/increase about 2%. This will vary with moisture content of the air/gas inside the tire. Tire pressure while operating can be ignored until the internal temperature reaches a minimum of 157deg F. A different pressure change ratio applies to altitude change, that too may be ignored. This another reason to not operate tires at their minimum inflation pressure as listed in tire mfgrs. load/inflation charts' say you inflate tires to their minimum in the Colorado mountains, drive to the CA coast-now your tires may be underinflated.

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You won't know what to set the cold pressure at until you weigh all 4 corners of the MH.

Best to weigh when fully loaded ready to travel and fuel tank full. Fresh water tank full or just a few gal, however you will travel with it.

Black & gray tanks with only a small amount in them.

 

After weighed use tire chart for PSI needed to carry for the highest side and then add 5 PSI.

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Is there a generally accepted ratio between cold and running pressures?

As dw8ite said no. Usually differences are around 10-20 psi. Could be a little more or a little less. Lots of factors that go into the running pressures - tires, loads, ambient temperature, sun/shade, cold pressure, etc.

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  • 4 weeks later...

You won't know what to set the cold pressure at until you weigh all 4 corners of the MH.

Best to weigh when fully loaded ready to travel and fuel tank full. Fresh water tank full or just a few gal, however you will travel with it.

Black & gray tanks with only a small amount in them.

 

After weighed use tire chart for PSI needed to carry for the highest side and then add 5 PSI.

According to the RMA=Rubber Manufacturers Association, ch 4 RV tires, page 55 says, in part:

However, never use inflation pressure lower than
specified by the vehicle tire placard, certification
label or owner’s manual. Nor should inflation
pressure exceed the maximum pressure molded on
the tire sidewall.
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We're picking up our new (to us) motorhome on Saturday -- a 1999 Beaver Contessa DP. First on my list of add-ons is a TPMS and I have a TST 507RV unit set to arrive on Friday.

 

There's a youtube video showing setup. One step references the tires' running pressure. I think cold tire pressure setting is pretty easy to obtain. Is there a generally accepted ratio between cold and running pressures?

 

Thanks,

 

 

Larry

Well, yes-in a way. Even the experts have difficulty calculating that though: https://www.physicsforums.com/threads/tire-pressure-changing-due-to-temperature.735541/

For our purposes DW8its is correct.

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According to the RMA=Rubber Manufacturers Association, ch 4 RV tires, page 55 says, in part:

However, never use inflation pressure lower than
specified by the vehicle tire placard, certification
label or owner’s manual. Nor should inflation
pressure exceed the maximum pressure molded on
the tire sidewall.

 

I don't think the Rubber Manufacturers Association know how much my MH weighs on front or rear axles that alone each corner.

Nor do they know like some owners I have changed the size on it. From 235/80/22.5 too a 275/70/22.5

And while doing that the tires changed from a G too a H rated.

Nor do a lot of owners know, if the used MH they bought have the OEM size tire the Placard refers to.

 

My Placard says 100 PSI front & 90 PSI rear.

My 4 corner weight for present G670 tires need only 85 PSI front or rear by the tire manufacture chart.

The tires have a maximum on the sidewalls of 125 PSI.

But my rims have a maximum of 120 PSI stamped on them.

 

So the best is still to weigh all 4 corners and follow the tire manufacture on what PSI to use.

I do that and add 5 PSI. That has worked good for my G670's that are 9 years 8 months old with no problems.

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Read the entire chapter, it explains what you are asking about much better than I could. My MH placard displays original tire size, your's should. I like my Goodyear G670, 275/70R22.5 tires too, mine are 8 now.

Footnote; Goodyear is a member of the RMA.

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