Correct Tire Pressure

[EddieMac]

I own a Class A motorhome equipped with Goodyear 255/70R22.5 G670 RV Unisteel. Load Range H, radial tires. I weighed my loaded Coach on a CAT Scale with the following weights, Steer Axle 9080 lbs, Drive Axle 13080 lbs. The Owners Manual contains a Load Inflation Table

PSI 80 85 90 95 100 105 110 115 120

Single 4190 4370 4550 4675 4895 5065 5205 5400 5510(H)

Dual 3970 4110 4275 4410 4455 4610 4675 4915 5070(H)

As a new motorhome owner, I am unsure about the correct pressure for each tire. Using the weight for each axle and above referenced chart, the correct pressure for the steer axle would be (9080 divided by 2 = 4540 lbs per tire) 90 psi. The correct pressure for the drive axle would be (13080 divided by 2 = 6540, divided by 2 tires = 3270 lbs per tire) 80 psi. Intuitively, this doesn’t seem right.

Would you please enlighten me on this subject?

Thanks for your assistance.

[wolfe10]

Eddie,

You are correct with two caveats:

Your answer ASSUMES perfect left/right weight distribution-- not likely with most coaches.

The tire manufacturer's chart gives the MINIMUM PSI for a given weight.

So, until you get individual wheel position weights, you need a "fudge factor" to account for unequal weight distribution. Once you have individual wheel position weights, go to your tire manufacturer's load chart with the HEAVIER WHEEL POSITION ON EACH AXLE. It's PSI is then used on ALL tires on that axle.

And, because you don't want to have to worry about PSI every time you add weight-- fill with fuel and a big shopping trip at Walmart, you need to add a little to compensate for more than that weight you measured.

So, most add 5 PSI to compensate for left right weight imbalance and 5 PSI to give you a little safety cushion (over minimum PSI for a given weight)-- as long as this PSI does not exceed the tire or wheel maximums.

And as soon as you are able to do individual wheel position weighing, you MAY be able to lower PSI a little if your left/right weight distribution is close.

Conclusion, I would run 100 front/90 rear until you have individual wheel position weights.

BTW, that is cold PSI-- NOT at a specific temperature, but before driving at whatever ambient temperature you are in.

Brett

[Guest Wayne77590]

Bret is right. My personal opinion for the rear could be 85 psi as that gives you a 840 pound fudge factor.

Also take into mind when you travel that ambient temperature, when tires are cold after sitting for a few hours varies by 2% for every 10 degrees of temperature change. Also consider altitude changes if you travel through mountainous areas. For every 1000 feet there is approximately a 0.48 psi change. Going up in temperature increases pressure, going up in altitude increases pressure . So if you set them as Brett states, or 85psi rear, you will not have to adjust for either temperature or altitude change unless you go to Kilimanjaro. That's the fudge factor.

Example: (Based on your weight)

Manufacturer's Recommended Weights and Pressures

Column---------1--------2--------3--------4-==-----5---------6---------7--------8---------9

Psi Change ---80------85-------90------95------100-------105------110------115------120

Single---------4190---4370----4550----4775---4895-----5065------5205----5400----5510(H)

Dual-----------3970---4110----4275----4410---4455-----4610------4675----4915----5070(H)

Your Wgt

Per Tire

Single Front----------------------------------------4540

Rear Dual-----3270

Psi----------------------------------------------------4455----4610-----4675

Single------------------------------------------------10 lbs---135 lbs--355 lbs

Fudge

Psi------------3970----4110

Dual----------700 lbs-840 lbs

Fudge

It appears to me, and I have been mistaken before, that your rear duals set at 80 psi is ample for your weight of 3270 per tire. I arrived at that figure dividing your total rear weight by four.

For the front, at 90 psi you only have a fudge factor of 10 pounds, Now DW can fix some shortcake an you could put on 10 pounds real quick (lol). At 95 psi you have a fudge actor of 135 pounds of weight per tire, and at 100 psi a fudge factor of 355 pounds. AT a 355 pound fudge factor you can invite one of the in-laws along. (lol-again)

Personally I would run the rears at 85 psi, but that is because I like the fact that I'm not running at Mfg minimum, and that gives me that fudge factor. I would run the front tires at 100 psi for the 355 lb fudge factor. A little rougher ride, if noticeable.

Running at the higher psi give you the fudge factor that you do not have to worry about changes in temperature and altitude. Any changes that may be encountered would still allow the tires to be within Mfg's specifications. Running at the bare minimum of 80 psi, you would be putting in or taking out air to maintain 80 psi, but at 85 psi, a 2 or 3 pound psi drop would still allow you to be within specifications.

If I don't have this right, someone let me just delete it.

Happy trails.

This post was edited by poster.

[tireman9]

Brett is right. My personal opinion for the rear could be 85 psi as that gives you a 700 pound fudge factor.

Also take into mind when you travel that ambient temperature, when tires are cold after sitting for a few hours varies by 2% for every 10 degrees of temperature change. Also consider altitude changes if you travel through mountainous areas. For every 1000 feet there is approximately a 0.48 psi change. Going up in temperature increases pressure, going up in altitude decreases pressure. So if you set them as Brett states, or 85psi rear, you will not have to adjust for either temperature or altitude change unless you go to Kilimanjaro. That's the fudge factor.

Here is a link to more detailed info on Elevation (Altitude) change.

Brett offers good advice on the minimum suggested inflation pressure. If you want more detail you can check this link.

[rhuffhines]

Question- I check my tire pressure when cold ( before the day's drive) and find one tire 3 lbs lower than I usually run. I don't have the equipment at the park to add air. Is it OK to add air at the fuel station. Like you measure the tire after you have driven- then add 3 lbs to that reading.

Will that be acceptable ?

[wolfe10]

Question- I check my tire pressure when cold ( before the day's drive) and find one tire 3 lbs lower than I usually run. I don't have the equipment at the park to add air. Is it OK to add air at the fuel station. Like you measure the tire after you have driven- then add 3 lbs to that reading.

Will that be acceptable?

Yes, that will get you close. Even better, add a little more than that and recheck in the morning before driving.

[rhuffhines]

Thanks Brett- sounds like good advice