Tire plus sizing

[CWtheMan]

Because plus sizing is done so often I feel discussing the proper procedures is necessary. NHTSA has approved provisions that should be followed when changing your tires from the Original Equipment (OE) size to another size that does not conform to the approved load inflation charts for the OE tires.

IMO when an owner uses replacement tires, that are not approved by the trailer manufacturer as being appropriate replacements, it does not relieve the owner of the responsibility to identify those unapproved replacements.

When seeking plus sized replacement tires, the basic tire industry standards are quoted here – in part - from the US Tire Manufacturers Association (USTMA); “Never choose a replacement tire of a smaller size or with less load-carrying capacity than the OE tire size at the specified vehicle tire placard pressure.”

The word size, in that text, means designated size. A designated size in tire industry terms includes the prefix identifier, when used. Therefore, size, such as ST225/75R15 includes its design, ST = special trailer. That’s the primary reason why reputable tire retailers will refuse to install LT tires when the OE tires are clearly ST tires.

Increasing a tire’s load capacity by going to the next higher load range is not considered a plus size replacement. The tire designated size does not change because the physical size of the tire did not change. The higher load range will use the identical load inflation chart. The load capacity is not part of the tire’s designated size.

Here is how the plus size procedure – in part – goes. Let’s say you’re going to use something simple, like replacing your ST205/75R14 LRC tires with ST205/75R15 LRD tires. The replacements are physically a little larger than the OE tires. You must first determine their fitment into the wheel well. Then, if used on a tandem axle trailer, that they have sufficient clearance between the axles. Next you will need a new wheel/rim size with load/pressure ratings compatible with the new tire designated size. (It’s recommended to use steel valve stems with trailer tires).

Once you have your replacements assembled and properly installed on your trailer you must find a load inflation chart for the replacement tires. Let’s say your trailer’s certification label has a recommended 50 PSI inflation value for your OE tires. With that they provided 1760# of load capacity. At the same 50 PSI the replacements will provide 1820# of load capacity. But, you got the replacements for more load capacity reserves. So you decide to set them at 60 PSI because your new rims are rated above that value and the 60 PSI will provide 2040# of load capacity.

You’re not done yet. NHTSA allows the use of auxiliary tire placards - they can be made by you – to be displayed adjacent to the OE tire placard. You can put the new tire size and your recommended tire inflation pressures on it. You should also make a pen and ink notation in your trailer’s owner’s manual, showing the same information you provided on the auxiliary tire placard.

Note: There are new tire sizes and load capacities showing-up almost daily. When finding a new load inflation chart to use for this post I saw inflation recommendations for a ST215/80R16 LRE (2910# @ 80 PSI). I guess we can thank RVIA for all these new sizes and load ranges the manufacturer’s are providing. They are much needed to keep pace with the 10% load capacity reserve recommendations RVIA made. While I’m on that subject. Remember, tire load capacity reserves are measured from the vehicle certified GAWR values. In other words, you will have 520# of load capacity reserves from some ST235/80R16 LRE tires fitted to 6000# GAWR axles properly inflated to 80 PSI. More when the axles are not carrying their maximum load.

[xrated]

Very good information sir.....thank you for posting it!

[jasin1]

Because plus sizing is done so often I feel discussing the proper procedures is necessary. NHTSA has approved provisions that should be followed when changing your tires from the Original Equipment (OE) size to another size that does not conform to the approved load inflation charts for the OE tires.

IMO when an owner uses replacement tires, that are not approved by the trailer manufacturer as being appropriate replacements, it does not relieve the owner of the responsibility to identify those unapproved replacements.

When seeking plus sized replacement tires, the basic tire industry standards are quoted here – in part - from the US Tire Manufacturers Association (USTMA); “Never choose a replacement tire of a smaller size or with less load-carrying capacity than the OE tire size at the specified vehicle tire placard pressure.”

The word size, in that text, means designated size. A designated size in tire industry terms includes the prefix identifier, when used. Therefore, size, such as ST225/75R15 includes its design, ST = special trailer. That’s the primary reason why reputable tire retailers will refuse to install LT tires when the OE tires are clearly ST tires.

Increasing a tire’s load capacity by going to the next higher load range is not considered a plus size replacement. The tire designated size does not change because the physical size of the tire did not change. The higher load range will use the identical load inflation chart. The load capacity is not part of the tire’s designated size.

Here is how the plus size procedure – in part – goes. Let’s say you’re going to use something simple, like replacing your ST205/75R14 LRC tires with ST205/75R15 LRD tires. The replacements are physically a little larger than the OE tires. You must first determine their fitment into the wheel well. Then, if used on a tandem axle trailer, that they have sufficient clearance between the axles. Next you will need a new wheel/rim size with load/pressure ratings compatible with the new tire designated size. (It’s recommended to use steel valve stems with trailer tires).

Once you have your replacements assembled and properly installed on your trailer you must find a load inflation chart for the replacement tires. Let’s say your trailer’s certification label has a recommended 50 PSI inflation value for your OE tires. With that they provided 1760# of load capacity. At the same 50 PSI the replacements will provide 1820# of load capacity. But, you got the replacements for more load capacity reserves. So you decide to set them at 60 PSI because your new rims are rated above that value and the 60 PSI will provide 2040# of load capacity.

You’re not done yet. NHTSA allows the use of auxiliary tire placards - they can be made by you – to be displayed adjacent to the OE tire placard. You can put the new tire size and your recommended tire inflation pressures on it. You should also make a pen and ink notation in your trailer’s owner’s manual, showing the same information you provided on the auxiliary tire placard.

Note: There are new tire sizes and load capacities showing-up almost daily. When finding a new load inflation chart to use for this post I saw inflation recommendations for a ST215/80R16 LRE (2910# @ 80 PSI). I guess we can thank RVIA for all these new sizes and load ranges the manufacturer’s are providing. They are much needed to keep pace with the 10% load capacity reserve recommendations RVIA made. While I’m on that subject. Remember, tire load capacity reserves are measured from the vehicle certified GAWR values. In other words, you will have 520# of load capacity reserves from some ST235/80R16 LRE tires fitted to 6000# GAWR axles properly inflated to 80 PSI. More when the axles are not carrying their maximum load.

Has this changed since you posted it in 2018? I thought you said it wasn’t allowed.. so to go with a plus size GY endurance that fits the wheel well and exceeds the load ratings....I just have to make an auxiliary tire placard?

[CWtheMan]

Has this changed since you posted it in 2018? I thought you said it wasn’t allowed.. so to go with a plus size GY endurance that fits the wheel well and exceeds the load ratings....I just have to make an auxiliary tire placard?

The above information is correct. Everything with plus sizing needs approval (s). Anytime a replacement tire with a designated size that is different from what is displayed on the vehicle certification label, plus sizing has happened and the certification label is no longer valid (A recommended cold inflation pressure has not been established).

A great many RV trailer owners randomly change designated tire sizes with complete disregard for the field. The field being someone that services the owners tires in their absents of a new owner of the trailer.

[jasin1]

The above information is correct. Everything with plus sizing needs approval (s). Anytime a replacement tire with a designated size that is different from what is displayed on the vehicle certification label, plus sizing has happened and the certification label is no longer valid (A recommended cold inflation pressure has not been established).

A great many RV trailer owners randomly change designated tire sizes with complete disregard for the field. The field being someone that services the owners tires in their absents of a new owner of the trailer.

I guess I’m a little slow because when I read the post. All 9 paragraphs, particularly the 8 th paragraph,you seem to be explaining the process for changing from a 14” tire to a 15” as an example
And that nhtsa allows the use of plus size as long as you don’t go to a smaller tire or load capacity.. and you document it with a auxiliary tire placard and note it in the owners manual.I’m not trying to prove you wrong or prove that I’m right.
I respect people with more knowledge then I have on a subject.
I’m just someone who won’t give up until I’m given a “ reasonable” answer.
It’s hard for me to accept when someone says “ you can’t” and when I ask why ? “Just because”
I never would have made it in the military lol

[jasin1]

I guess I’m a little slow because when I read the post. All 9 paragraphs, particularly the 8 th paragraph,you seem to be explaining the process for changing from a 14” tire to a 15” as an example
And that nhtsa allows the use of plus size as long as you don’t go to a smaller tire or load capacity.. and you document it with a auxiliary tire placard and note it in the owners manual.I’m not trying to prove you wrong or prove that I’m right.
I respect people with more knowledge then I have on a subject.
I’m just someone who won’t give up until I’m given a “ reasonable” answer.
It’s hard for me to accept when someone says “ you can’t” and when I ask why ? “Just because”
I never would have made it in the military lol

Any way I will let it go now, I have enough info and facts to make a decision. Thanks for your time sir

[CWtheMan]

I guess I’m a little slow because when I read the post. All 9 paragraphs, particularly the 8 th paragraph,you seem to be explaining the process for changing from a 14” tire to a 15” as an example
And that nhtsa allows the use of plus size as long as you don’t go to a smaller tire or load capacity.. and you document it with a auxiliary tire placard and note it in the owners manual.I’m not trying to prove you wrong or prove that I’m right.
I respect people with more knowledge then I have on a subject.
I’m just someone who won’t give up until I’m given a “ reasonable” answer.
It’s hard for me to accept when someone says “ you can’t” and when I ask why ? “Just because”
I never would have made it in the military lol

Paragraph # 2.

IMO when an owner uses replacement tires, that are not approved by the trailer manufacturer as being appropriate replacements, it does not relieve the owner of the responsibility to identify those unapproved replacements.

[jasin1]

Paragraph # 2.

IMO when an owner uses replacement tires, that are not approved by the trailer manufacturer as being appropriate replacements, it does not relieve the owner of the responsibility to identify those unapproved replacements.


It does not relieve the owner of the responsibility to IDENTIFY those unapproved replacements
I’m interpreting that to mean( by reading the rest of your post) that it is my responsibility to choose tires that meet or exceed the original oem tires in load specifications and if I go to a larger size and choose a different rim with higher specs or a larger tire then I have to provide the auxiliary placard.
I’m doing that as the owner of the rv. I feel most of these rules are written for the seller of an original rv at first point of sale

[jasin1]

Paragraph # 2.

IMO when an owner uses replacement tires, that are not approved by the trailer manufacturer as being appropriate replacements, it does not relieve the owner of the responsibility to identify those unapproved replacements.

Do these rules ever get revisited after being written down in the book of tire rules? I’m not trying to argue or be bullheaded. It seems almost silly not to be able to build a stronger link in a chain.
I feel like regulations like these are compiled together in 1000 page long reports that nobody challenges.

[jasin1]

Paragraph # 2.

IMO when an owner uses replacement tires, that are not approved by the trailer manufacturer as being appropriate replacements, it does not relieve the owner of the responsibility to identify those unapproved replacements.

Well CW...you have a wonderful evening and a Merry Christmas or happy holiday..I enjoyed discussing this tonight and look forward to more on the subject.

[CWtheMan]

Do these rules ever get revisited after being written down in the book of tire rules?

After a revisit to this post it appears I didn't answer this question.

Tire industry standards are often supported by NHTSA mandates. My information for vehicle manufacturer approval for replacement tires comes from such a mandate.

The wording may differ slightly from manufacturer to manufacturer but the bottom line will support the following statement in the Keystone owner's manual which is mandated to be there by NHTSA. It's also mandated to be in other documents/files industry wide.

Tire Size:

"To maintain tire safety, purchase new tires that are the same size as the vehicle’s original tires or another size recommended by the manufacturer. Look at the Tire and Loading Information label, or the sidewall of the tire you are replacing to find this information. If you have any doubt about the correct size to choose, consult with the tire dealer."

Because this is an industry wide mandate, they mention seeking help from the tire dealer. On the automotive side of the house, vehicle manufacturers provide tire retailers/dealers with a listing of all approved replacement tire sizes for all vehicles each manufacturer builds. That practice does not exist/happen with RV trailer tires.

[jasin1]

After a revisit to this post it appears I didn't answer this question.

Tire industry standards are often supported by NHTSA mandates. My information for vehicle manufacturer approval for replacement tires comes from such a mandate.

The wording may differ slightly from manufacturer to manufacturer but the bottom line will support the following statement in the Keystone owner's manual which is mandated to be there by NHTSA. It's also mandated to be in other documents/files industry wide.

Tire Size:

"To maintain tire safety, purchase new tires that are the same size as the vehicle’s original tires or another size recommended by the manufacturer. Look at the Tire and Loading Information label, or the sidewall of the tire you are replacing to find this information. If you have any doubt about the correct size to choose, consult with the tire dealer."

Because this is an industry wide mandate, they mention seeking help from the tire dealer. On the automotive side of the house, vehicle manufacturers provide tire retailers/dealers with a listing of all approved replacement tire sizes for all vehicles each manufacturer builds. That practice does not exist/happen with RV trailer tires.

I appreciate you taking the time to respond
Have a good day sir and thanks

[Roscommon48]

My thought on upgrading/changing rv tires are to go to the next TRAILER lettered tire with the highest weighting. I try to keep it simple.


You can google tire weights all you want...very easy for us old folks.

[CWtheMan]

My thought on upgrading/changing RV tires are to go to the next TRAILER lettered tire with the highest weighting. I try to keep it simple.


You can google tire weights all you want...very easy for us old folks.

Just a note; when using a replacement tire with a higher load range letter, such as ST225/75R15 LRC to a ST225/75R15 LRD, the tire designated size has not changed. Therefore, the newer, higher load range tire, is not a "plus size". At 50 PSI they both provide an identical load capacity. Everything above that is considered load capacity reserves.

[Sarge2]

CW, my Passport has ST 205 75 14's... they are Trailmaster D rated with a spd rating of 65 at 65lbs air pressure...
I was thinking of GY Endurance ST 215 75 14's 8ply Load rated D same as my originals, but have a higher lb load than others and speed rating of 87mph...
Not that I would driver over 65mph towing it but having a bit higher speed rating would at least make me feel a bit more comfortable towing at 65mph than a tire with a speed rating of 65...
any real "comfort zone" there or simply mind over matter?

[CWtheMan]

CW, my Passport has ST 205 75 14's... they are Trailmaster D rated with a spd rating of 65 at 65lbs air pressure...
I was thinking of GY Endurance ST 215 75 14's 8ply Load rated D same as my originals, but have a higher lb load than others and speed rating of 87mph...
Not that I would driver over 65mph towing it but having a bit higher speed rating would at least make me feel a bit more comfortable towing at 65mph than a tire with a speed rating of 65...
any real "comfort zone" there or simply mind over matter?

All I can say is the following is a tire industry standard; "Always refer to and follow the vehicle manufacturer's replacement tire recommendations."

That same information is mandated to be in your trailer's owner's manual. For me to answer it any other way would get at least a half dozen popcorn poppers going.

[jasin1]

All I can say is the following is a tire industry standard; "Always refer to and follow the vehicle manufacturer's replacement tire recommendations."

That same information is mandated to be in your trailer's owner's manual. For me to answer it any other way would get at least a half dozen popcorn poppers going.

:lol::lol::lol: Forgive me for being hardheaded

[Sarge2]

All I can say is the following is a tire industry standard; "Always refer to and follow the vehicle manufacturer's replacement tire recommendations."

That same information is mandated to be in your trailer's owner's manual. For me to answer it any other way would get at least a half dozen popcorn poppers going.

Maybe I should have said "off the record" or PM me... I get where you are going... call that Vicarious Liability.. ;)

[CWtheMan]

Maybe I should have said "off the record" or PM me... I get where you are going... call that Vicarious Liability.. ;)

It's also explained in the following NHTSA quote supporting industry standards.

Industry standards generally form the basis for demonstrating product safety and quality before courts, regulators, retailers, consumers and others."