Determining Tire Pressure

Use the vehicle information printed on the door. It has been determined by the manufacturer using GVWR.

Thanks, that actually seemed the best choice to me as well.

Who do you think spent the most time and money determining how much weight a tire will carry at a certain pressure?

I will go by tire companies chart any day.

The vehicle maker gives you the psi to use based on the OEM tires. They cannot possibly fathom what each owner will replace those OEM tires with and makes that information invalid once the OEM tires are replaced unless replaced with the identical tire in every respect. Come on and lets think a little please. No vehicle maker is sticking their neck out on replacement parts obtained by the end user. Once the OEM tires are replaced use the tire makers inflation chart. Holy moly sometimes I just SMH.

The tire company puts a recommended pressure on the tire for a reason!

The correct way is to go to a scale and weigh each tire position to see how much weight each tire is carrying. Then go to the tire manufactures web site and look for the inflation table, find your size tire, look at the weight you recorded, and the chart will tell how much air you need to carry that weight. I usually add 5 PSI for safety reasons. If you have different weights on the same axle, use the highest weight to determine the PSI you need.

A sample load and inflation chart
Load chart sample

The vehicle placard is correct for the size and load range tires specified by the maker (as printed on the placard), generally assuming a fully loaded vehicle. For a class C motorhome, that's often fairly close to the truth, so these are a decent starting point.

Tire manufacturers do give tables of pressure based on load. A more accurate approach than using the placard is to weigh the axles with the motorhome fully loaded and ready to go (or preferably each corner, and use the higher end of the axle when there's a difference) and use the manufacturer's chart to determine the minimum inflation. Most would go a few pounds above this to provide a little cushion for extra loading. (Of course, one should not go over the tire's maximum pressure rating, nor the wheel's maximum pressure rating--which may be lower than the tire's maximum, particularly if tires of a different size or load rating than originally specified are used.)

The pressure stamped on the sidewall of the tire is the maximum pressure for the tire, and only applicable at or near the maximum rated load. Having too high a pressure for the loading can lead to abnormal tire wear, a harsh ride, worse traction and braking performance, and often poor vehicle handling. Underinflation can lead to much the same thing, and also causes the tires to heat up more which can cause them to fail.

To summarize: the best way is to weigh the loaded motorhome and look up in a table for the size and type of tire. (It does not need to be for the exact tire manufacturer or model; the sizes and load ratings are standardized.) A usually acceptable way is to go by the placard on the door. Putting the maximum rated pressure in all the tires, as stamped on the tire sidewall, is generally speaking not a good idea.

The tire company puts a recommended pressure on the tire for a reason!

Actually, I've never seen a "recommended" pressure on tires.

What you normally find is the maximum allowed pressure for the tire. But it may not be necessary or even desirable to inflate your tires to the maximum.

Any given tire's load rating is determined by how much air is in it. Generally speaking, the more air in it, the more load it will endure, up to its maximum rating.

For example, I have Cooper tires on my RV. As I recall they can be inflated up to like 90 PSI. But my rims are only good to 65 PSI. My RV owner's manual says I should inflate the rear tires to 62 PSI and the front tires to 58 PSI. The critical issue is what is the load rating of the tire at 65 PSI, since I can't inflate them to the full maximum rating.

Steve

Tyler0215 wrote:
The tire company puts a recommended pressure on the tire for a reason!

Tire companies put the max pressure on a tire, not the recommended pressure. The max pressure is the pressure required if the tire is operated at maximum loading.

Weight the axles and set tire pressure according to the tire manufacturer's load chart. I add an extra 5 psi for margin.

If you call your tire manufacturer with the weight of your vehicle
their customer service will give you the correct PSI.

The door PSI is for MAX.

So if ypu are under MAX your tires are over inflated.
A good indicator of over inflation is poor handling and braking.
Does your RV feel like if is on ice?
If so ypu most likely have over inflated tires.

FYI on E450 chassis vehicles:
Using Michelin's chart:
225/75/16E

65 PSI Single (Front) equates to the GAWR of the 4600# front axle E450 used through 2007 (chassis year)
and
75PSI works out to GAWR of the 5000# axle used from 2008 up.

Duel = axle lbs
70 PSI for Duels = 4440 lbs
75 PSI for Duels = 4660 lbs
80 PSI for Duels = 4940 lbs

If your E450 Sticker says:
GVWR 14050 then
Front GAWR 4600 lbs.
Rear GAWR 9450 lbs.
65 / 80 PSI

That would be MAX.
If your weight is under that then you are over inflated.