Basic question. "What tires do I need?"

 There seems to be some confusion on tire selection for RV applications. As a Tire Design Engineer with over 45 years of experience. I would like to offer some basic advice as not everyone has worked with tires or RVs.
First, you need to confirm the original Type, Size, and Load capacity as provided by the RV company. A Certification Label with GVWR (Gross Vehicle Weight Rating) and GAWR (Gross Axle Weight Rating), Tire Type, Size, Load Range, and Inflation, along with the VIN and some other info, was applied according to Federal Law to all vehicles sold. Since about 2004 Class-As have had the label near the driver's Left Elbow. It may be on the floor or the interior wall of the motorhome. Class B and Class C would have the label on the "B" pillar, AKA Door Jam. Towables had the label applied to the Outside, Driver side front 1/4 of the RV length.
Tire Type (identified by letter before the numbers in the tire size) could be "LT", such as LT235/80R16, or "ST", such as ST205/70R15, or "P", such as P235/75R15 or have no letters in front of the numbers, such as 345/80R22.5.
Tire Size is ALL the numbers such as 235/75R15 or 295/80R22.5 or 195/75R15
Load Range is a letter such as "C", "D", or "E" on Class-C RV. letters "C" & "D" are most likely found on Class B or Van type motorhomes. ST type tires on trailers will most likely have the letters C, D or E. Class-A Motorhomes probably have the letters starting with E, F or G and can go on up to H or J. Some smaller trailers may have "P" type tires with the inflation number on the tire in the 30's and up to 50 psi if they also have "XL" as part of the tire size nomenclature.

An important bit of information you need to have is a truck scale reading when the RV is fully loaded (all the water, food, clothes, toys, fuel, and other "stuff" etc, you expect to carry.) The scale reading should NEVER exceed the GAWR found on your Certification label. That weight should also be below 95% of the "Max Load" shown on the tire sidewall or found on the Certification Label.
Example if GAWR is 4,400# which is the max on that axle then the scale reading for the 2 tires on that axle should be no greater than .95X4,400 or 4,180# for the 2 tires or 2,090# for each tire. Hopefully, the tire "Max Load" number molded on the tire sidewall is greater than 2,090#. Actual load should ALWAYS be lower than the GAWR and ALWAYS be lower than MAX Load specified on the tire when fully inflated.

TPMS or Tire Pressure Monitor System. Good ones run $300 and up, depending on the number os tires you should monitor. Some folks say, "I check inflation at every fuel or rest stop. That's good to do, but I bet you drive a lot more miles down the road than you do while in a fuel stop or rest area and clearly you are more likely to get a puncture while driving than while you are stopped.

If you ask people who have had a tire failure because of a puncture, ask them what the total cost was in time and money for them to replace the punctured tire.

A question on Max inflation on the wheel

 Question and post from Doug, a reader of this blog

Roger Marbles' tire dissertation…. one item I constantly see miss is the allowed tire pressure on the steel wheel. Each wheel is stamped with a code from the National Wheel and Rim Assn. I just happen to have acquired one of their spec sheets. The wheels on my Class A are stamped (embossed) “AF” which means the maximum pressure is 95lbs. Is that ambient cold or maximum hot? That I don’t know and it isn’t explained. I presume it is cold or ambient. So, is there a benefit in buying a higher maximum load tire or not?- If the wheel can’t take up to, say 120lbs of pressure! (Load range F tire).

 My reply:  Unless you are fine-tuning a race car handling, the inflation we are talking about is ALWAYS the “Cold” inflation of the tire. RVtravel.com has a section on “Maintenance & Repair Archives.” Just click in the blue header to find any of my tire-related posts. Or you can click on this to get to my posts. You can also do a “word search” if you do not see the topic you are looking for. BUT in this case I previously covered. If you need greater load capacity, just changing the tire does not increase the GAWR spec for your RV, as the GAWR spec covers the tire, wheel, hub brakes, nuts and bolts, and axle and mounting brackets, etc. So just increasing the load capacity of the tire will not result in an increase in the max load capacity of the RV.

Hi  Roger, I am well aware of the tire ambient air psi, which I have read in your posts numerous times. My specific concern is with the steel wheel vs the rising pressure when traveling down the road. If the wheel is allowed to hold 95 psi, is the rising rolling pressure increase above that safe for the steel wheel? With an F load tire, the MINIMUM pressure to support the load is – say 105 lbs cold, and a wheel at only 95 lbs cold – already over the max steel wheel psi – what happens when the pressure increases even higher to 120-130 or more psi from rolling heat buildup? 

Doug, I thought I was clear but the ONLY pressure we are concerned with is the “COLD” pressure. This applies to both the tire and the wheel. In your example, something is wrong. Either the tire or wheel is too small or has too low a load rating or the load being placed on the tire & wheel is to great. If you know the actual load on the tire & wheel and the inflation shown in the tables for that size tire and wheel is greater than that specific tire capacity, then you are overloading the system and need to reduce the load and or increase the size tire or the tire Load Range (Psi). Example below:
Assume we have a 225/75R19.5 LR-E tire. That tire is rated for 3,195 @ 80 PSI cold in a Single application. Also assume the wheel is rated for 80 psi. If you go on a scale and learn the tire load is 3,445# you MUST change something. Either change to a stronger wheel & Tire or lower the load. A LR-F tire could support 3,640# BUT that would require 95 PSI rated wheels. Just changing the tire to LR-F and increasing the inflation DOES NOT increase the load capacity of the wheel, which is limited to 80 PSI cold.