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Old 04-28-2012, 07:51 PM   #1
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Tire Pressure - Help Please

I finally got my RV weighed. Maybe someone can help me with the tire pressures I should be running.
I checked the Firestone chart and it doesn't help worth a darn and I'm still trying to figure it out.

Here's some info on my rig:

W22 Workhorse Chassis
Firestone Tires 255/70R22.5
GVWR 22000
GVWRF 8000
GVWRR 14500

Scale Weights:

Front Left. 3703lbs
Front Right 3527lbs
Rear Left Dual 6790lbs or 3395lbs each
Rear Right Dual. 6922lbs or 3461lbs each


Firestone chart only starts at 3970 lbs and 80psi for each dual and 4190 lbs at 80psi for single wheel.

This is my first time dealing with this and I'm feeling a little dumb!
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Old 04-28-2012, 07:55 PM   #2
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Never go lower than the lowest pressure on the tire chart. Looks like that's what you need to run
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Old 04-28-2012, 08:03 PM   #3
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Agreed, 80psi is the minimum.. I would prolly run 80 rear, 85 front in your case..
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Old 04-28-2012, 08:07 PM   #4
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I agree. I would run 80-85 on all tires and see how the ride is. Nothing lower than 80.
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Old 04-28-2012, 08:19 PM   #5
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Low pressure

I have kind of the same situation with my Bounder but I decided to run my fronts at 115 and rears at 100. (Michelin XRV's) I think the Michelin chart recommended 100 and 85 psi in my case but the tires felt mushy at that pressure.

At higher pressures there is less heat build up due to flexing sidewalls and more safe pressure margin at the expense of a little bit rougher ride. It might effect fuel mileage too but when the best you get is around 8 how much difference is it going to make?


I highly recommend a TPMS.
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Old 04-28-2012, 11:29 PM   #6
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Quote:
Originally Posted by dogpatch View Post
I finally got my RV weighed. Maybe someone can help me with the tire pressures I should be running.
I checked the Firestone chart and it doesn't help worth a darn and I'm still trying to figure it out.

Here's some info on my rig:

W22 Workhorse Chassis
Firestone Tires 255/70R22.5
GVWR 22000
GVWRF 8000
GVWRR 14500

Scale Weights:

Front Left. 3703lbs
Front Right 3527lbs
Rear Left Dual 6790lbs or 3395lbs each
Rear Right Dual. 6922lbs or 3461lbs each


Firestone chart only starts at 3970 lbs and 80psi for each dual and 4190 lbs at 80psi for single wheel.

This is my first time dealing with this and I'm feeling a little dumb!
Dogpatch,
Go to the Firestone web site and click on the Load/PSI Tables.

The minimum for your tires according to the chart is 80 psi for 4190 single tires, and 80 psi for 3970 pounds, so you have the minimums correct. However the chart goes up to 105 psi without getting into Load Range "D"or "H". So for the standard tires, the high end is 5065 pounds at 105 psi for a single, and 4610 pounds for each dual at 105 psi.

So for your weight you would take the heaviest corner weight for that axle and for your front that would be 3703 x 2 (7406 total axle weight) and for the rear the heaviest axle weight would be 6922 x 2 (13844 total).

Using the heaviest corner weight per axle.
255/70R22.5
Single = 80 psi supports 4190 lbs (X2=8380) -- you are 3703 (x2=7406) -- your fudge factor is 974 pounds on the axle (evenly distributed of course) before you need more air. (next load range in the chart)

Dual = 80 psi supports 3970 Lbs per tire (7940 Total) = you are 3461 each dual (6922 total) - your fudge factor is 1018 pounds evenly distributed across the axle before you need more air. (next load range in the chart)

Here's the kicker though. At 80 psi you are at the manufacturer's minimum recommend air (Sidewall of tire should tell you min and max pressures and loads) So if you should drop 20 degrees in temperature you could loose 4% of the air due to temperature decrease (2% for every 10 degrees.) Should you also drop in altitude by 2000 feet you would loose an additional 0.96 psi (say 1 psi) If you failed to check your tires on your next move you would be at 75.8 psi(76 rounded) and that may not be a good thing. (to low)

Now, if you inflate your tires to 85 psi you will not only increase your fudge factor, but should the same scenario appear with temperature and altitude, and you forget to check your tires before you move, you will be sitting at 80.6 psi (81 rounded) and within the tire specifications.

Wow! Hope I didn't mess that up. Anyone?

Edited: p.s., on the sidewall of the tire is a long string of numbers with a DOT ending in 4 numbers. Those four numbers are the week and year of the manufacture of the tire. Example: 2403 would be the 24th week of 2003. If your tires are older than 5 years you may want to consider replacing them. It's just $$$$$! Check with Firestone.

Also, you may feel that the handling is a little better with more air in the tire, but the lower pressures will give you a more cushy ride.

You could also do the "chalk test." Take a piece of white chalk. Draw a line across the treads of each tire. Move the RV for a couple rotations of the wheels and then look at the chalk line. Just the center being rubbed off - to much air. Just the edges rubbed off - to little air. Rubbed off all across - just right. (so I have been told) (You an look at the chalk mark on the ground to see what the distribution is also.)
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Old 04-29-2012, 02:31 AM   #7
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I agree with Wayne.
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Old 04-29-2012, 07:29 AM   #8
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Somewhere on your MH is the manufacture's sticker that shows the tire pressure that the suspensioins was designed to exprct. Here's the one from our MH:
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Old 04-29-2012, 08:54 AM   #9
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Quote:
Originally Posted by JJH3rd View Post
Somewhere on your MH is the manufacture's sticker that shows the tire pressure that the suspensioins was designed to exprct. Here's the one from our MH:
That's the part that confused me. The coach originally came with 235/80R22.5 tires. The last tire changeout, the previous owner put 255/70R22.5 tires. I believe this is Firestones's equivalent to the original size, however can't be certain. That's why I tried to look it up on Firestones's site.
On my Workhorse factory sheet it shows front 95psi and rear 85psi based on 235/80R22.5 tires.
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Old 04-29-2012, 09:31 AM   #10
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Previous owner probably had the same problem I had. Blowout on a weekend. The 255/70's are plentiful on the road where you might have a hard time finding 235/80's. Not one 235 available on a weekend in Jackson ms but you could get all the 255/70's you wanted. Besides they seem to be cheaper than the 235/80's and offer greater load range protection.
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Old 04-29-2012, 03:45 PM   #11
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Dogpatch,
Ignore everything for the 235/80's, as you are not using them anymore. You are using the 255/70's and that is the table/chart I posted about. You also cannot use the mfg's sticker as it also was designed with the 235/80's in mind. You have to go by the tire manufacture's information on the tire that you are using. You have already performed the more challenged task of getting the four corners weighed.

Are the special tires, like "G" or "H" rated? Even if they are not, you will be within tire specifications at 80 or 105 psi, but if you loose air from 80 psi you will be below the minimum, and if you set the pressure at 105 and it has increased when "cold" tire temperature is met you would be over specification. Setting for 85 through 100 would put you within specifications and you could loose or gain 5 pounds at the low or high end. The information I posted was for your heaviest wheel weight per axle.

Good luck.
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Old 04-29-2012, 11:39 PM   #12
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....and if you set the pressure at 105 and it has increased when "cold" tire temperature is met you would be over specification. ...

Good luck.
Say what? 110 PSI cold is the max.. there is no allowing for heating..
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Old 05-11-2012, 01:32 AM   #13
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I would start with the tires at the maximum PSI for them in dual mode and start from there. Underinflated tires are going to wear faster and run hotter and be a possible safety hazard and you are going to burn more gas. After 1000 miles you can tell from the wear pattern whether it is advisable to reduce the pressure in any of the wheels.

When you get to a G rated tire it is not only able to handle 110 PSI, it is designed to be used at that pressure. Lower pressures may not cause the bead to break free but it will cause a lot more flexing of the sidewall and a lot more scrubbing of the tire as it is driven down the road.
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Old 05-11-2012, 08:44 AM   #14
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Say what? 110 PSI cold is the max.. there is no allowing for heating..
Yes, 110psi is the max stamped on the sidewall of the tire. Heating is calculated into the tire by the manufacturer so if the tire is 110 psi cold it meets that maximum requirements of the manufacturer even if it is driven and the temperature increase the pressure. When arriving at destination and the temperature has increased by several degrees, and the elevation has gone up a couple thousand feet, the cold inflation may be over the 110 max psi stamped on the sidewall. It should be changed to read the correct cold tire max inflation of 110 psi if that is what one runs at. Cold inflation pressure is as the tire sits and comes to ambient temperature.

In the inverse, (decrease temp/alt) it depends on loaded weight as to the tire manufacturer's pressure specifications according to their table.

Sorry I missed the post a few days ago, I though it was a dead horse and didn't want to shoot it again.
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