I’m getting a lot of flack with my tire posts lately so I’m going to set the stage.
My comments here are Pros & Cons and will fall in-line with tire rules & regulations and tire industry standards. In other words, I’m not going to step over the safety line.
First off there is the question of who to seek information from when changing tire designated sizes. It’s always the vehicle manufacturer first. The following quote is from the Keystone generic 2020 owner manual.
“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.”
A tire’s designation is a primary factor in the tire size. ST235/80R16 is a designated size. ST235/85R16 is a designated size. Because they use different load inflation charts they are not interchangeable without vehicle manufacturer approval. That approval can be predetermined when offered as a vehicle manufacturer option.
Replacement tires all fall under the same basic industry standard which says, in part, that they must provide a load capacity equal to or greater than the OE tires provided. All new – post 2007 – standards will also mention that they must be of the same designated size. ST235/80R16 a designated size with four load range numbers, D – E – F & G. Speed ratings are provided by individual brand builders.
When going up in load range, wheels & valve stems must have the ability to support the increase in Load, PSI or Both.
Keystone has always built most of their trailers with very close axle spacing. Some are so close they will not allow even the slightest increase in tire overall diameter.
When a consumer goes out on a limb and arbitrarily uses unauthorized replacements, they should know the proper procedures to follow. (The procedure is the same for authorized or unauthorized plus sized tires.) NHTSA allows auxiliary tire placards when plus sized tires do not meet the standards of a load inflation chart for the OE tires. The auxiliary placard should list the new tire size and the cold inflation pressure needed for them to provide a load capacity at least equal to the load capacity the OE tires provided. The proper location for the auxiliary placard is adjacent to the original.
Example of an auxiliary tire placard. Trailer or Light Truck tires would have load range in place of load index.
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