The correct tire size comes from your 5th wheel’s certification label, the complete sidewall marking, and the weight each axle actually carries.
A tire one size bigger can still rub the fender on the first hard turn, and rim diameter alone says little about what fits — your trailer already documents the 5th wheel tire size it needs.
Every replacement must match on dimensions, construction, load capacity, load range, speed rating, wheel fitment, and inflation. Miss one and it may mount correctly while being wrong for the load underneath it.
Where The Required Tire Size Is Printed On Your Trailer
The original tire size, recommended cold inflation pressure, and axle ratings sit on the trailer’s certification label, usually on the driver’s side near the front. The owner’s manual repeats those numbers and adds the gross vehicle weight rating (GVWR) and each axle’s gross axle weight rating (GAWR).
That label is the starting point. NHTSA’s trailer testing guidance (TP-110T-03) states that the combined maximum load ratings of the tires on an axle must not be less than that axle’s GAWR. Two tires on a 6,000-pound axle therefore need at least 3,000 pounds of capacity each.
- Copy the exact size string off the label, not a similar-looking one.
- Note the load range listed for that position.
- Write down the recommended cold pressure before you shop.
Reading Your 5th Wheel Tire Size Off The Sidewall Marking
Every character in a sidewall marking does a specific job, and the complete string is what you match against the tire maker’s load and inflation chart. The “16” is only the wheel diameter.
Take ST235/80R16 123L as the example:
| Sidewall Code | What It Tells You |
|---|---|
| ST | Special-trailer tire designation |
| 235 | Nominal section width, 235 millimeters |
| 80 | Aspect ratio — sidewall height is 80% of section width |
| R | Radial construction |
| 16 | Nominal wheel diameter, 16 inches |
| 123 | Load index — a capacity code, not a weight in pounds |
| L | Speed symbol |
Load range and load index are separate items. Load range describes a tire’s load-and-inflation capability category; the load index is just a numeric code, so an “E” and a “123” are not substitutes. Michelin’s RV tire load and inflation tables pair the complete sidewall marking with the cold pressure that matches your weight, and Michelin tells users to match the whole string instead of guessing from one part of it.
How Much Load Does Each Tire Actually Carry?
The number that decides inflation is not the trailer’s rated weight but the load each axle end carries once the 5th wheel is packed for a trip. The heavier end sets the pressure for both tires on that axle.
Load the trailer the way you travel — fresh water, propane, gear, tools. Weigh each axle, and each wheel position if you can. Michelin’s guidance is to weigh each axle end separately and use the heavier side to set pressure, keeping both tires on that axle at the same cold psi.
Pressure also has a ceiling that isn’t the tire’s. The wheel’s approved rim width, bolt pattern, offset, diameter, and maximum air-pressure rating all have to be compatible, and the wheel manufacturer’s limit wins even when the sidewall permits more. Run the lower of what the tire, the wheel, and the trailer placard allow.
Check pressure cold — parked at least an hour, per NHTSA — because heat from driving raises the reading and can mask a slow leak. Inspect every tire before a trip for bulges, peeling, embedded objects, and uneven wear.
Four mistakes show up again and again:
- Buying by diameter alone and ignoring load capacity.
- Swapping a special-trailer (ST) tire for a light-truck (LT) one without confirming capacity, fitment, and manufacturer approval.
- Using the trailer’s GVWR instead of actual axle weights.
- Mixing radial and non-radial tires, which NHTSA lists as a practice to avoid.
Once the size, load range, and required capacity are pinned down, a tested roundup of tires for 5th wheel RVs is the fastest way to compare what meets those numbers.
Before you order, work through it in this order:
- Copy the full tire size and load range off the certification label.
- Copy the complete sidewall marking off the current tire.
- Weigh each axle fully loaded and note the heavier end.
- Look up the cold pressure for that weight in the maker’s table.
- Confirm the wheel’s width, bolt pattern, and maximum pressure rating.
Common Questions
Can I Use LT Tires Instead Of ST Tires On A Trailer?
Federal rules do not categorically ban light-truck or passenger-car radials on a trailer, as long as load capacity is properly adjusted for axle weight. Still, confirm capacity, wheel fitment, and manufacturer approval before swapping, and never mix radial and non-radial tires on the same trailer.
Should I Inflate Tires To The Pressure Printed On The Sidewall?
No. The maximum figure on the sidewall is a limit, not a target. The cold pressure you need depends on the load each axle end actually carries and the tire maker’s load-and-inflation table. Never exceed the wheel’s own maximum pressure rating either, since that limit can sit lower than the tire’s.
Why Should Both Tires On One Axle Run The Same Pressure?
Wheel-position loads are rarely identical, and cargo or tanks can make one side much heavier. Weigh each axle end separately and set pressure from the heavier side for every tire on that axle. Uneven pressure between two tires sharing an axle leaves the lighter-loaded tire overinflated for the work it is doing.
Sources
- Michelin Commercial Tires. “RV Tire Load & Inflation Tables.” Source of the load-and-inflation matching method, per-axle weighing guidance, and wheel pressure limits.
- National Highway Traffic Safety Administration. “TP-110T-03: Trailer Tire Testing.” Trailer testing guidance on tire maximum load rating versus axle GAWR.
- National Highway Traffic Safety Administration. “Interpretation ID 9679.” Addresses tire types for trailers and warns against mixing radial and non-radial tires.