Tire pressure question

Bruce H · July 14, 2018

I just bought a ( new to me ) 2006 Forest River Georgetown XL 326 DS. Getting ready to go fulltime. Before I start loading it I checked the load information chart on the wall. It says for tire size 245/70R19 SF 85 PSI dual rear and 90 PSI front. The actual tires are Michelin 235/80R22.5 XRV. 110 PSI front & rear. I assume the previous owners installed the larger tires to support more weight. The GVWR is 22,000 lbs. Because the tires are larger and before I start loading the MH, should I inflate the tires more towards the 110 psi range? Thoughts & opinions would be greatly appreaciated.

Thanks

TreyandSusan · July 14, 2018

The thing that you need to do is get wheel position weights with your RV loaded as soon as you can. Then you will really know what to do with the newer tires.

It is hard to know exactly which tires were original. A good/probable guess would be the Goodyear G670, however that would still be a problem because this specific tire changed its load/inflation significantly over time. There were two versions of this size for this Goodyear model tire (now there is only 1 version). So, knowing the exact loads:inflation for the original tire are not possible, because we do not know the original tire.

Therefore, making a comparison based on the original tires is not possible.

So plan B... The large majority of the time the RV manufacturer for motorized vehicles provides recommended inflation pressure by providing enough inflation for the tires to exceed the load capacity for the GAWR (front and rear). Therefore, knowing the front and rear GAWR would help to provide the same estimation that the manufacturer usually does.

The Load inflation table for this Michelin tire is...

For example...

If the front GAWR is 7500 lbs (a guess) and we consult the Michelin Engineers Load inflation table (above) for the given single tire we see that the equivalent inflation for the new front tires would be 80 psi.

And if the rear GAWR is 14500 lbs (a guess) and we consult the Michelin Engineers Load inflation table (above) for the given dual tire we see that the equivalent inflation for the new rear tires would be 90 psi.

These inflation pressures would be sufficient to carrying the maximum rated load of the axle with these new tires, assuming the loads across the axle are evenly distributed.

Since you did not provide the given GAWR from the Federal certification label inside the motor home, the front and rear GAWR above are just guesses to provide an example to see how to determine what the manufacturer probably did to provide inflation recommendations and what you could do for the new tires.

It would be very good to get your loaded Motor home weighed by wheel position to see if there are any side to side differences and to more accurately determine actual loaded inflation.

Sometimes there are differences side to side and sometimes it is easy to overload axles. The manufacturer assumes that you will not overload the axle and that the weight on the axle is evenly distributed. Unfortunately often these two conditions do not prove to be true once the vehicle is loaded with our stuff. RVs regularly have significant side to side weight differences because of the nature of an RV being a living space. And RV owners often do not understand the need to be aware of weight so they load them up to the point that they may exceed the various vehicle safety ratings.

Bruce H · July 14, 2018

The GAWR for the front is 7500 lbs. The GAWR for the rear is 14500 lbs. So I'm looking at 85psi in front and 90 psi in the rear, according to the chart. Being that these are larger tires, 95psi in front and 100 psi in rear. Would this be a fair assumption?

Bruce H · July 14, 2018

Trey, I take it this chart you posted is not in lbs. So let me retract my statement. 85psi front 90psi rear.

FL-JOE · July 14, 2018

In a perfect world getting a 4 corner weight done on your full loaded coach would be ideal. Then, as suggested, you simply consult the correct manufacturers tire chart that applies and adjust your psi accordingly.

Unfortunately every town doesn't have a place where you can drive in and get a 4 corner weigh in. My suggestion would be to run your fully loaded rig across a CAT truck stop scale. Use those axle weights and the tire chart to adjust your psi until you can find a place for a 4 corner weight.

Bruce H · July 14, 2018

Thank you all for your suggestions. Makes perfect sense.

TreyandSusan · July 14, 2018

The chart is in pounds and psi. psi across the top and pounds for the tire loads.

I think you may not be understanding what I am saying so I will try again. This chart is for the "bigger" newer tires not the older original tires. What I have done is to show you what the manufacturer would have likely done for the sticker if they would have put these "bigger" tires on originally. "Bigger" depends on what you are talking about but that is a whole different discussion. what we care about now is these tires and what would be the equivalent manufacturers recommended inflation pressure had they used these tires. For motorized vehicles this sticker inflation pressure is almost always set to provide enough load to carry the gross axle weight rating (GAWR).

These "bigger" tires have pretty much the same load capacity as the original tires, maybe just a little bit more. Who knows why they installed these tires because they would have had to also upgrade the rims as well.

Another thing that is needed to understand is that what is on the sidewall of the tire is not necessarily the inflation pressure you are supposed to use to inflate the tires on a given vehicle. What the sidewall says is the maximum load capacity of the tire and the inflation needed to carry that load. For this specific tire that would be Single 4675 lbs and in order to carry 4675 lbs you would need 110 psi. With Dual tire configuration the load is reduced so that is why the Dual max load is 4410 for each tire and to carry that load you would need 110 psi. The original tire 245/70R19.5 the load capacity was probably Single 4540 at either 110 psi or 100 psi depending on the specific tire that was used in 2006 when the chassis was produced and provided to Forest River

For the inflation table below, the single line is for each tire in single configuration. The Dual line is for each tire in Dual configuration. The Dx2 is for the wheel position for the Dual pair or the dual load times 2 since there are two tires for a dual wheel position.

The front axle would use the single line because the tires are in single tire configuration (one tire on each end of the axle). For a fGAWR of 7500 lbs., each of the two tires would carry half of the axle load or 3750 lbs. The inflation for one tire in single tire configuration to carry 3750 lbs is 80 psi. (3860 lbs). 75 psi would not be enough because it will only carry 3645 lbs. 85 psi would carry 3975 lbs.

80 psi would likely be the sticker inflation pressure if these "bigger" Michelin XRV 235/80R22.5 tires were used.

The rear axle is dual tire configuration or 4 tires across the whole axle. With a rGAWR of 14500 that would mean that one pair of the dual tires would be assumed to carry no more than 7250 lbs (14500/2=7250) OR each dual tire would carry 3625 lbs (14500/4=3625). So, the inflation needed for these dual tires to carry the rGAWR would need to be 90 psi. 85 psi would be a little too low (7230 lbs for the dual pair or 3615 lbs for each dual tire).

The equivalent recommended sticker inflation pressures for the front and rear GAWR using this "bigger" tire would be 80 psi for the front and 90 psi for the rear.

Getting a weight at a platform scale is a good thing but there is an issue with using axle weights and adjusting the inflation to fit the axle weights. RVs have a higher tendency to have greater side to side weight differences. This means if you assume that the weight at each end of the axle is half of the measured axle weight and inflate to that number, then you could be underinflating a tire (or tires in dual configuration) on one side of the axle.

The safest thing to do until you can get wheel position weights is to go by the RV manufacturers recommended inflation pressure which would normally be found on the Federal Certification Sticker. In your case this recommended inflation was based on a completely different tire than you currently have and that tire had a completely different load capacity and inflation table. With the new ("bigger") 235/80R22.5 tires you now have these equivalent sticker inflation pressures which would be 80 psi on the front axle tires and 90 psi for the rear axle tires. This is what you asked for in the original post.

One advantage of getting platform scale weights (CAT scale) is that if you find an axle weight that is at or exceeding the capacity you can increase the inflation pressure by 5-10 psi. Of course it would be good to try and reduce the load on the axle. You now know that because you have an RV it is more likely that you could have a greater difference in axle end (wheel position) weights. So, if your axle weight is near, at, or exceeding the GAWR then you would need to increase your inflation pressure IF the tire load capacity and inflation chart allows you to go higher, which this tire does allow.

On a motor home, when you are able to get weighed by wheel position then you would use the heaviest wheel position on that axle for the determination of the inflation pressure. You would need to use the correct inflation table for that specific tire and then you would find the minimum cold inflation pressure based on that heaviest wheel position for that axle. Then for reserve capacity add 5-10 psi and inflate the tires on that axle to that pressure. (Trailer tires are done a little differently)