Tire Pressure—Am I Wrong?

[Ramona]

I am a little disconcerted that my RV Dealer let me drive my new coach off the lot with what, by my calculations, are very under inflated tires. Since I am hoping that I am the one misunderstanding tire pressure, I thought I would check with you all .
Tires are Goodyear 245 70R19.5G 133/132L. On the tire it says "Max PSI 110 lbs".
The chart inside says "Front GAWR 7500 lbs, 92 PSI. Rear GAWR 13,500 lbs, 92 PSI.
Without boring you with the details, I figure we are well below the GAWR. I was planning on keeping the tires at about 90 PSI (and setting up TPMS for that).
I was shocked when I checked cold tire pressure and it was 78 PSI.

The Goodyear Tire Inflation Chart indicates that a tire pressure of 80PSI may be acceptable (see attached).


Before I add 12 lbs of pressure to each tire, tell me if I am missing something that would account for the dealer inflating to only 78 lbs.
Thank you!

[Blufox]

Chart is good, however the only way to accurately determine tire pressure is to have each axle weighed individually. Then, interpolate these weights into the equation to arrive at the correct tire pressure for each axle.

[Podivin]

Yep, unless the fact that you got the coach weighed is one of the boring details you left out, the best thing for you at this point is to inflate to 92 psi. Use that until you get the coach weighed and then refer to the inflation tables for the proper inflation.

[Arch Hoagland]

Ramona...I have about five tire gauges. I use the two that match readings all the time.

Have you checked the pressures with another tire gauge?

The dealer may have set the pressure on a 100 degree day and when you checked them maybe it was 50 degrees. That would make a few pounds difference.

It's good that you are checking them.... a lot of people don't.

[Ramona]

Originally Posted by Blufox

Chart is good, however the only way to accurately determine tire pressure is to have each axle weighed individually. Then, interpolate these weights into the equation to arrive at the correct tire pressure for each axle.

Yes, I'd like to do that but no where close to here for it. Also, since I am confident we are under weight, without doing a weight for each tire, is it really that helpful?

[edge68474]

I would not run under 8o lbs. I have the same size tires, Toyo, and the minimum from Toyo says 80. I am close to being maxed out when loaded both front and rear. I run 85 lbs in the front and 95lbs in the rear.
I would check them cold in the morning personally.


P.S. And it looks like we have the same size rig at 7500 front and 13,500 rear

[AKIQPilot]

Seems like at 78psi you are slightly under inflated for the max GAWR of each axle. There is a pretty good chance that when you drove the coach off the lot you were well under the max GAWR for each axle.

As everyone above posted already. Weigh your coach and adjust tire pressure from there. I too use 3 different gauges (plus my TPMS gauges) to check tire pressure. All of my gauges are generally within 2% of one another.

[Ramona]

Originally Posted by Arch Hoagland

Have you checked the pressures with another tire gauge?

The dealer may have set the pressure on a 100 degree day and when you checked them maybe it was 50 degrees. That would make a few pounds difference.

Good point about the ambient temperature.
I have not checked the tire pressure with a second gauge, but the one I have is a good quality Tire Minder.

[edge68474]

Ramona. I just saw your last post. Put everything in your MH that you would normally carry then get it weighed. Mine was full of water and gas plus everything else. Some folks don't carry a full tank of water, some don't carry any. my view is to have some water anyway. 4 corner weight is good to know but at least get each axel weighed , then when you can get each corner weighed. Have fun.

[Blufox]

Originally Posted by Ramona

Yes, I'd like to do that but no where close to here for it. Also, since I am confident we are under weight, without doing a weight for each tire, is it really that helpful?

YES, while your present total weight may not exceed GCWR, having the coach weighed will
•. Confirm gross weight for front axle ( both R & L)
•. Confirm gross weight for the rear axle again, both right and left.
•. Confirm weight for each axle individually as you coach is presently loaded.

Knowing if you have an overweight axle affords you the opportunity to adjust the load to balance weights.

I had my coach weighed (all axles individually) September of 17. Making mods, adding appliances ( washer & dryer) and a well stocked tool box and some spare parts, I elected to again have all axles weighed September 18. Weights changed❗️ I now through calculations had a new set of tire pressures for all . Plus 5lb in each front and plus 10lb for each tire on the rear.
This insures proper inflation when tires heat up and protects for premature tire wear and possible failure. BTW, in the unlikely event of a front tire blow out. . . Press on the accelerator until control is maintained then brake cautiously .

[tuckmf]

First post here.
As a road racer we adjusted tire pressures by temperature reading across the tire tread. If the middle was hotter than the edges you decreased the tire pressure 1 - 2 lbs and vice versa when the tires were hotter on the outer edges. We used nitrogen to minimized moisture in the tire and make temps more predictable. Wouldn’t this work on a RV?

Mike

[Blufox]

Mike,
Excellent point and well taken. My TPMS characteristically gives high temp and high pressure on my RF. Using my heat gun, proved the TPMS was incorrect. Measuring across the tread was done early on to serve my curiosity. Now, usually once a day while at a rest stop. Nitrogen while available and expensive is something I have elected not to do. Usually the 5w drivers opt for nitrogen as history shows, experience blowouts. Cannot remember a days drive where I have not seen 5w or tow behind on the shoulder with tire failure.

[Hooligan]

Originally Posted by Ramona

Yes, I'd like to do that but no where close to here for it. Also, since I am confident we are under weight, without doing a weight for each tire, is it really that helpful?

Very simple to find a "CAT Scale" at a truck stop near you in Albuquerque. Drive on the scales with front axle on one segment, rear on the other. Will give total weight, and individual axle weights. Not as good as individual wheel weight but for $10-15 you know what you are working with. Use same pressure on both ends of the axle and chart pressure is the Minimum required pressure for that weight.
https://catscale.com/cat-scale-locator/

[Ray,IN]

I use the pressures listed on the Federal Tire Placard. Tire manufacturers state to never run less than the recommended pressure than the Federal Tire Placard in the vehicle, see page 4 of https://www.goodyearrvtires.com/pdfs..._nov112014.pdf