Tire pressure......

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golftrip

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Does anyone have some type of tire pressure chart I could use as a guide for tire pressure on my 1977 Dodge Diplomat II Motorhome?  The tires are an odd size, 17.5 R..I just got two new tires and want to make sure I have the pressure correct.  I am thinking 65-70 PSI, but would feel better if I had some advice from forum members.  Thank you!
 

Gary RV_Wizard

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You need the chart provided by the manufacturer of the tire fopr that specific brand and model of tire. There is no universal chart.

Most of the major tire manufacturers have their inflation charts online on their website. What brand are yours and what is the full tire model designation, i.e. tread width, tire profile, type(usually R these days) and rim size plus the load range or ply rating.  Example, 235/80R22.5 10 ply (or perhaps Load range G)
 

golftrip

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The tires are Bridgestone R273SWP.? They are 8R 17.5, load range E, 10 ply, max pressure is 85psi cold.
The rims are 17.5 inch.
 

Gary RV_Wizard

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I found the tires specs on the Bridgestone-usa site but no inflation table for it.  Strange that a company that makes a lot of noise about tire safety (since the Explorer-Firestone debacle) makes it hard to find inflation data.  I guess you will have to go to a Bbridgetsone dealer and ask - they are supposed to have them for each tire model.  Before you do, you need to know the weight carried by each tire, preferably by weighing each wheel separately. But both tires on the same axle must be inflated to the same pressure, so it's good enough to weigh each axle, divide by 2 and round up to the nearest 100 lbs.

Tite inflation is by weight range. The 85 psi maximum shown on the sidewall will also give a weight with it, i.e. xxxx lbs at 85 psi. A tire pressure of 85 psi will support that much weight. Less weight allows less pressure, but you never want to be under on pressure. Excess pressure gives a hard ride and may cause the center of the tire to wear faster than the edges, but will avoid premature tre failure.

If your weights are near the maximum weight rating, go with the 85 psi max pressure. I would estimate that "near" means within 250 lbs on this tire. If more than 250 lbs under the max weight rating, you could probably go with 10 psi less. I wouldn't go much lower than that without seeing an actual tire inflation table from Bridgestone, which will give exact inflations for each weight range. 65 psi is probably a minimum for the tire, but that sheer gueswork on my part.  As I said, you never want to be low on pressure.
 

BruceinFL

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Why mess around with inflation charts? Why not just go ahead and pump em up to 85 psi cold and you'll be OK on all the wheels?
 

Ron

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BruceinFL said:
Why mess around with inflation charts? Why not just go ahead and pump em up to 85 psi cold and you'll be OK on all the wheels?
Biggest reason is we do not want to be guessing at something that can adversely affect our safety as well as others.  Guessing at tire pressures is like playing the poker sometimes you might win and sometimes you could very well loose your life.  If inflation pressures were not important then there would be no reason for inflation charts.  Having over inflated tires is almost as bad as under inflated tires.  FOLLOW your tire manufacturers recommendations and the pressure on the side of the tire is not a recommendation it is telling you what pressure is acceptable at maximum load the tire is designed for. 

Have you by any chance watched the Michelin Tire Safety Video that was previously mentioned on this forum.  If not I would highly recommend watching it.  The tire Safety Video can be found here: http://www.michelinrvtires.com/michelinrvtires/other/RvVideos.jsp

 

Gary RV_Wizard

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Why not just go ahead and pump em up to 85 psi cold and you'll be OK on all the wheels?

Here are some reasons:

(1) Ride comfort - too hard a tire gives a REALLY rough ride;

(2) Handling - too hard a tire can make the front end skittish because the contact patch is narrow, mostly only the centerline of the tread;

(3) Tire wear - too hard and the tire rides mosty on the center of the tread and thus wears unevenly

That said, I would use max inflation if I had no weight information at all. But there's really no excuse for not getting at least axle weights. As Ron says, its not something one should guess at.
 
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