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Old 01-18-2015, 10:13 PM   #15
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Richard, thanks for your suggestion. I think it makes good sense to start with an 80lb fill for each tire and then see how it rides. Since we will be staying at KOA's during our journey I won't be loading the rv down with much water and will of course be dumping any grey and black water at the KOA before each day's drive. In other words, I don't think we'll be overloaded.

Thanks gentlemen for all of your suggestions. I really appreciate the help.
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Old 01-19-2015, 11:59 PM   #16
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After filling all the tires to 80psi I finally found the placard with the motorhome stats. Here they are:
GAWR: Front - 4880 Rear - 7500
GVWR - 12,300
PSI Cold Single: 60
PSI Cold Double: 50

So how should I proceed from here? Should I change the tire pressure to 60 in front and 50 in the rear?

I have not begun the drive yet, but plan to begin on Wednesday.

Many thanks,
Kevin
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Old 01-20-2015, 12:03 AM   #17
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Are your present tires the same size and weight rating as the original?
If so, then yes, change the pressure, but remember that those charts are ONLY for the RV with it's original tires and the RV weight at it's maximum legal weight. Which is why you need to weigh the rig and consult the tire manufacturers charts.
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Old 01-20-2015, 12:24 AM   #18
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The placard calls for 7.5x16LT and the tires on there now are Firestone LT 215/85R16. The Firestone tires have these specs:
Max Load Single: 2680 lbs
Max Load Dual: 2470 lbs
Followed by the number 550 for both single and dual, then
Max Pressure 80psi.

I have not been able to find the Firestone chart for this tire yet...
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Old 01-20-2015, 12:33 AM   #19
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What model are they, not just the size and brand. The model makes a difference too.
Being an LT tire they may not be intended for RV use in which case you probably won't find a load/pressure chart. If that's the case then 80 psi might be the correct answer.
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Old 01-20-2015, 12:37 AM   #20
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They are the Transforce HT. I just found what may be the weight/tire pressure info for the tire at https://www.tirerack.com/tires/tires...num=185R6THTV2

Ah nope, just the basic specs that are embossed on the tire. Where is the detailed inflation chart for this thing I wonder?
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Old 01-20-2015, 12:52 AM   #21
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Looks like they don't have a weight/pressure chart as they are a Light Truck tire and they don't usually need/have the charts for them. In that case I'd run them at 80 psi.
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Old 01-20-2015, 12:55 AM   #22
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All right Mr. D. Thanks for your help, I really appreciate it.
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Old 01-20-2015, 01:06 AM   #23
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BTW: I'm sure others will have something else to say, they just aren't up right now!
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Old 01-20-2015, 06:01 AM   #24
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Quote:
Originally Posted by Newrider View Post
After filling all the tires to 80psi I finally found the placard with the motorhome stats. Here they are:
GAWR: Front - 4880 Rear - 7500
GVWR - 12,300
PSI Cold Single: 60
PSI Cold Double: 50

So how should I proceed from here? Should I change the tire pressure to 60 in front and 50 in the rear?

I have not begun the drive yet, but plan to begin on Wednesday.

Many thanks,
Kevin
Made picture of my filled in spreadsheet using the tire and car data you gave . Came to higher pressures even if I put reserves on zero.
This makes me suspect the oem tires where D-load with 65 psi as AT-pressure.
Will google for the sises in D load to see if that gives closer answer to the F60 R50 psi you gave.

Edit: searched the sises and gave LRD 110 loadindex= 2337 lbs a tire maxload, and this is not enaugh to carry the front GAWR of 4880 wich must be by law, so conclusion E-load are the original tires.
Then if you calculate it with american formula wich gives to much deflection.
Only for front to check and gave 70 psi so fully loaded the 60/50 is not enaugh.
And because most motorhomes come close to the GVWR because of the way they are build up and normally loaded, you have to use the GAWR's to determine the needed pressure. I even add a reserve to the load , but not to much so it does not give bumping.

The 550 you read from tires is 550kPa = 5,5 bar =rounded 80 psi.
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Old 01-20-2015, 07:37 AM   #25
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I had the same tires on my old Class A with the same GVWR, but 27' long. They work great as small MH tires. Unfortunately, I can't remember what tire pressure I used.
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Old 01-20-2015, 08:34 AM   #26
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Quote:
Originally Posted by Newrider View Post

After filling all the tires to 80psi I finally found the placard with the motorhome stats. Here they are:

GAWR: Front - 4880 Rear - 7500
GVWR - 12,300

PSI Cold Single: 60
PSI Cold Double: 50

So how should I proceed from here? Should I change the tire pressure to 60 in front and 50 in the rear?

I have not begun the drive yet, but plan to begin on Wednesday.

Many thanks,
Kevin
Quote:
Originally Posted by Newrider View Post

The placard calls for 7.5x16LT and the tires on there now are Firestone LT 215/85R16. The Firestone tires have these specs:

Max Load Single: 2680 lbs
Max Load Dual: 2470 lbs


Followed by the number 550 for both single and dual, then
Max Pressure 80psi.

I have not been able to find the Firestone chart for this tire yet...
Kevin,

Based on the numbers you supplied above, the 7.5X16LT is simply stating that your wheel will handle a tire designed to go onto to a rim that is 7.5 inches by 16 inches. The LT stands for Light Truck (Tire).

So when you look at the numbers, your coach has a MAX weight of 12,300 lbs including cargo and people.

Your front axle can handle 4800 lbs. (4880 typo error)

Your rear axle can handle 7500 lbs.

Ignore the CIP numbers as they are not relevant other then when the coach was new.

If you inflate your current tires to 80 lbs on a single axle (front) they are good to a max of 5360 lbs. (2680 X 2). The rear dual tires will handle a total of 9880 lbs. (2470 X 4).

So you do have a little play to reduce the psi of the tires and still carry the max weight of 12,300 lbs.

You could start out with 80 psi and if the ride is TOO harsh then try 75 lbs but I would not go any lower than the 75 lbs.

Remember that ride is dependent on other mechanical items in your chassis besides tires.

Also, once you have your coach fully loaded with people and cargo you could stop at a CAT Scale or someplace that has a scale to see what it weighs and that will give you MORE information to work with.

Dr4Film ----- Richard
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Old 01-20-2015, 09:46 AM   #27
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Hi Kevin,
One note of caution: Do not let any air out of your tires until they have cooled overnight. Never let air out of a warm/hot tire.
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Old 01-21-2015, 06:03 AM   #28
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@ ChasA

This is not that the valve or tire is destroyed by that at that moment, but its because you dont know how warm the tire inside is at that moment.
The only reliable measurement is cold, and that is when inside tire temp is the same as outside tire. But by sunshine on those black energy absorbing tires the tire inside can get higher , so be ware of that, even when cold measurement.

When pressure is to low when you measure warm, you are allowed to fill up ,
at least to adviced cold pressure, but even a bit more to expected warm pressure in normal conditions.
But then you are filling cold air to warm air , so pretty inacurate to determine.
also you dont know how warm the tire inside is.
Can be about 110 degrees from only driving, wich highens up the pressure at 65degr F by about 10 to 11% , but also up to boiling point of water ( 200degr F ??) by the heat of the brakes transported trough the rimms to the tire inside, wich highens up the cold pressure of 65 degr F by about 35%.
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