Start with your trailer's GVWR (printed on the VIN/certification label), divide it by the number of tires touching the ground, then add a 15–20% reserve. That result is the minimum load each tire must carry. Match it to a tire size and load range whose stamped "max load" meets or beats it. Example: a 7,000-lb tandem-axle trailer needs roughly 2,015 lb of capacity per tire.
The one calculation that picks your tire
Every correct trailer-tire choice comes down to one number: how much weight each tire has to hold. Here is the method, in order:
- Find your GVWR (Gross Vehicle Weight Rating) — the loaded weight your trailer is rated to carry, trailer plus cargo.
- Count only the tires on the ground. Single axle = 2, tandem = 4, triple = 6. The spare does not count.
- Divide: GVWR ÷ number of load-bearing tires = base minimum per tire.
- Add a reserve. Multiply by about 1.15–1.20 (a 15–20% cushion). RV-industry sources recommend a reserve because factory tires are often spec'd barely adequate for the axle, and uneven loading, heat, and age all eat into capacity.
- Match a real tire. Pick a size and load range whose sidewall max load equals or exceeds your target.
Worked out for common trailers:
| Trailer GVWR | Tires on the ground | Base min per tire (GVWR ÷ tires) | Target with ~15% reserve | Example ST size that meets it |
|---|---|---|---|---|
| 2,990 lb | 2 (single axle) | 1,495 lb | ~1,720 lb | ST205/75R14 Load Range C (1,760 lb) |
| 3,500 lb | 2 (single axle) | 1,750 lb | ~2,015 lb | ST205/75R15 Load Range D (2,150 lb) |
| 7,000 lb | 4 (tandem) | 1,750 lb | ~2,015 lb | ST205/75R15 Load Range D (2,150 lb) |
| 10,000 lb | 4 (tandem) | 2,500 lb | ~2,875 lb | ST235/80R16 Load Range E (3,520 lb) |
| 12,000 lb | 4 (tandem) | 3,000 lb | ~3,450 lb | ST235/80R16 Load Range E (3,520 lb) |
Notice the 10,000-lb row: the base minimum is 2,500 lb, but with a reserve the target climbs to about 2,875 lb — just past what an ST225/75R15 Load Range E (2,830 lb) holds. That is exactly why you size to the target, not the bare minimum, and step up when you land close to a tire's ceiling.
Where to find GVWR and your current tire specs
You do not need to guess any of these numbers — they are printed on the trailer and on the tire.
On the trailer: Look for the federal certification/VIN label, usually on the front-left (road-side) of the frame or on the tongue. It lists GVWR, the GAWR (gross axle weight rating for each axle), and the trailer's original tire size, load range, and cold inflation pressure. If any tire on the trailer is smaller or a lower load range than that label specifies, it is undersized.
On the sidewall: The size reads like ST205/75R15 (ST = Special Trailer). Nearby you will find the load range letter (C, D, E…) and a line that says "Max Load ____ lb at ____ psi cold." That max-load figure is the per-tire capacity you compare against your target. If you want a full walk-through of what each part of the size code means, see our trailer tire size explained guide, and for what the letter actually buys you, our trailer tire load range explained guide.
Common GVWR-to-load-range matches
Capacity depends on both size and load range, so there is no single "load range = X pounds" answer — a Load Range D in one size holds more than a Load Range D in a smaller size. Use the actual per-tire numbers below (representative radial ST values; the exact figure is stamped on your sidewall and varies a little by brand):
| ST tire size | Load range | Max cold PSI | Max load per tire | Per axle (× 2 tires) |
|---|---|---|---|---|
| ST175/80R13 | C | 50 | 1,360 lb | 2,720 lb |
| ST205/75R14 | C | 50 | 1,760 lb | 3,520 lb |
| ST205/75R15 | C | 50 | 1,820 lb | 3,640 lb |
| ST205/75R15 | D | 65 | 2,150 lb | 4,300 lb |
| ST225/75R15 | D | 65 | 2,540 lb | 5,080 lb |
| ST225/75R15 | E | 80 | 2,830 lb | 5,660 lb |
| ST235/80R16 | E | 80 | 3,520 lb | 7,040 lb |
| ST235/85R16 | E | 80 | 3,640 lb | 7,280 lb |
These max-load numbers only hold at the listed cold pressure, so inflating to the sidewall max is part of hitting the rating — details in our trailer tire pressure guide. We carry these ST sizes plus value brands like LANDSAIL, LEAO, FULLWAY, GOODRIDE, and ZEETEX in our trailer tires collection, and our Orlando, Florida warehouse ships free — usually 1–3 days across the Southeast.
Getting the details right: reserve, tongue weight, and speed
A few points keep the calculation honest:
- Why full GVWR is conservative (and that is good). In real towing, part of the trailer's weight rides on the hitch as tongue weight (often 10–15% on a bumper-pull), so the axle tires never actually see the entire GVWR. Sizing to full GVWR already builds in a cushion; the extra 15–20% reserve on top is the belt-and-suspenders margin RV sources recommend.
- Going up a load range is fine; going down is not. Moving D → E adds capacity, but only if your wheel is rated for the higher pressure. Never drop below the load range on the certification label. If you are weighing trailer-specific ST against light-truck rubber, our ST vs LT tires for trailers guide covers the trade-offs.
- Capacity does not change your speed rating. Standard ST trailer tires are rated for a maximum of 65 mph unless the sidewall or manufacturer states otherwise. A bigger or higher-range tire does not let you tow faster.
- Re-torque after mounting. New wheels settle — re-check lug nut torque at 10, 25, and 50 miles.
Matching the wheel matters too: the rim has to accept the tire's diameter and handle the load range's pressure. We stock aluminum trailer wheels and dual-bolt-pattern options (like the Rhino Predator) in our trailer wheels collection.
FAQ
Can I just put a higher load range tire on to be safe?
Yes. Going up a load range (for example D to E) adds capacity and is a safe move, as long as the tire size still fits and your wheel is rated for the higher cold pressure that range runs. Never go below the load range printed on your trailer's certification label.
Does the spare count when I divide GVWR by the number of tires?
No. Only count the tires carrying load on the ground — 2 for a single axle, 4 for tandem, 6 for triple. Size your spare to match the others so it is a true replacement if you need it.
How fast can I tow on ST trailer tires?
Standard ST trailer tires are rated for a maximum of 65 mph unless the sidewall or manufacturer specifies higher. Some newer ST tires carry a higher speed symbol, but only when inflated and loaded exactly per the maker's chart. Sizing for load capacity does not raise the speed rating.
Sources: Tire and Rim Association load and inflation tables; U.S. DOT / NHTSA tire selection and GVWR/certification-label requirements (FMVSS 571.110/120); RVIA reserve-capacity guidance; and manufacturer load-inflation tables (e.g., Kenda Loadstar, Carlisle, Goodyear). Exact per-tire figures are stamped on the tire sidewall and vary slightly by brand and construction.