Community Forums

Mahlon · 06-26-2005, 05:05 PM

Hope someone can help me sort through the morass of information to help me determine the correct tire pressures for me. Here is the pertinent data:

THe coach: 2003 Newmar Scottsdale 3270 / W22 chassis

The tires: Goodyear G670 245/70R19.5, load range G.

The weights (per my friendly neighborhood CAT scale):

3410 | | 3260

6520 || || 7060

The placard in the coach says 100 psi all around. According to the Goodyear pressure chart (which can be found here )the tire pressures at this loading should be 70 psi front, 80 psi rear. At full 22K GVW (8000 fr, 14500 rear) again according to the table the pressures need only increase to 90 psi front, 80 psi rear.

So, who's right, what should I set the pressures to, and why does the manufacturer call out the obviously too high number of 100 psi? Since we're taking a two-week trip with the coach starting this week, I need to sort this out ASAP. Thanks for your help.

Mahlon · 06-26-2005, 05:05 PM

Hope someone can help me sort through the morass of information to help me determine the correct tire pressures for me. Here is the pertinent data:

THe coach: 2003 Newmar Scottsdale 3270 / W22 chassis

The tires: Goodyear G670 245/70R19.5, load range G.

The weights (per my friendly neighborhood CAT scale):

3410 | | 3260

6520 || || 7060

The placard in the coach says 100 psi all around. According to the Goodyear pressure chart (which can be found here )the tire pressures at this loading should be 70 psi front, 80 psi rear. At full 22K GVW (8000 fr, 14500 rear) again according to the table the pressures need only increase to 90 psi front, 80 psi rear.

So, who's right, what should I set the pressures to, and why does the manufacturer call out the obviously too high number of 100 psi? Since we're taking a two-week trip with the coach starting this week, I need to sort this out ASAP. Thanks for your help.

wilanddij · 06-26-2005, 06:34 PM

Went through this a while ago myself. Consensus on the responses I got back was to go with the pressure recommended by the tire manufacturer for your weight, and add between 5 and 10 psi for margin.

Hooligan · 06-26-2005, 10:04 PM

Originally my MH placard had tire pressures that agreed with the tire mfg for front and rear GAWR. In Nov. 2000 Freightliner initiated a recall campaign for the placard, and provided replacements that indicated the max inflation pressure for the tires. "To comply with Federal Motor Vehicle Safety Standard 110.Tire selection and rims."
The original placard gave GAWR at 90psi. New wording was GAWR at 100 MAX psi.
Michelin Rep stated that the placard shows max pressure but to use inflation table for better traction, wear and ride.
Hooligan

RV Wizard · 06-26-2005, 11:27 PM

I too would recommend you use the mfg's tire pressure chart for the weight you have on board + a little for a safety margin. Remember that the tire pressure to weight ratio is obtained for a speed of 65 mph. For every 1 mile per hour over that you need to derate the tire by 50 lbs of weight carrying capacity. Check them each day cold and if you buy a lot of stuff alon the way add a little more psi to the tires. Enjoy and drive carefully.

knute · 06-27-2005, 02:51 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by RV Wizard:
For every 1 mile per hour over that you need to derate the tire by 50 lbs of weight carrying capacity. </div></BLOCKQUOTE>

I'm skeptical about this statement. Let me restate for my simple mind. If I run at 75 mph, then each tire carries 500 lbs less load, 6 tires x 500 lbs/tire = 3000 lbs less load in the MH.

I've never seen nor heard of this "rule of thumb". Can some authority be cited for the origin?

edgray · 06-27-2005, 05:43 AM

Originally posted by MAHLON:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">The weights (per my friendly neighborhood CAT scale):

3410 | | 3260

6520 || || 7060 </div></BLOCKQUOTE>
Since you are susposed to inflate all tires ON THE SAME AXLE to the pressure needed for the heaviest loaded tire, may I suggest you try to rebalance your rear axle, by moving things around if possible. While almost no coaches are perfectly balanced side-to-side, I think 540 more on the RR is a concern. At the very least, it will force you to run the rear duals at higher pressure, which you MAY be able to soften if you can redistribute the weight. Have a wonderful trip! ED

Mahlon · 06-27-2005, 01:37 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">... may I suggest you try to rebalance your rear axle, by moving things around if possible. While almost no coaches are perfectly balanced side-to-side, I think 540 more on the RR is a concern. At the very least, it will force you to run the rear duals at higher pressure, which you MAY be able to soften if you can redistribute the weight.... </div></BLOCKQUOTE>

I had the same thought. The coach was weighed loaded with all fluids full plus two passengers, pretty much the same as how it will be on the road. The only remaining cargo will be food (on right side, not good), one std 8 year old boy (insignificant weight, plus won't like riding in one of the cargo compartments

) plus clothes (on left side, better but not much weight). Since the bulk of the imbalance on the rear is caused by the weight of the galley, rear slide, & generator all on the right side, there isn't much I can do about it. Wish there was.

I think I'll go with Goodyear's recommendations for tire pressure that are consistent with the coach/chassis gross weight ratings. Certainly there will be sufficient "headroom" and the coach will ride better than it would at 100 psi all around (I hope).

Thanks for the advice, folks.

Kirk · 06-27-2005, 02:52 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by RV Wizard:
For every 1 mile per hour over that you need to derate the tire by 50 lbs of weight carrying capacity.

I'm skeptical about this statement. Let me restate for my simple mind. If I run at 75 mph, then each tire carries 500 lbs less load, 6 tires x 500 lbs/tire = 3000 lbs less load in the MH. </div></BLOCKQUOTE>

The RV Wiz is exactly right. The fact is that tires have a speed rating was well as one for weights. There was information printed in Motorhome magazine in the past few months on that same subject. There is a letter in the DOT code that lists the speed rating and for most large tires that speed is 65mph.

Mahlon · 06-27-2005, 05:12 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kirk:

The RV Wiz is exactly right. The fact is that tires have a speed rating was well as one for weights. There was information printed in Motorhome magazine in the past few months on that same subject. There is a letter in the DOT code that lists the speed rating and for most large tires that speed is 65mph. </div></BLOCKQUOTE>
Goodyear G670s are rated for 75 mph, but I wouldn't dream of running them that fast. 65 - 68 mph seems to be the sweet spot for my rig; any slower and the Allison transmission can't seem to decide whether or not to shift into overdrive, any faster and fuel economy goes downhill FAST.