You're shopping for trailer tires and you see two nearly identical options — same size, same load range, similar price — but one says "ST205/75D15" and the other says "ST205/75R15." One letter is different. D or R.
That single letter changes how the tire is built, how it rides, how it handles heat, and how long it lasts. If you're making the wrong choice for your towing style, you might be overpaying for features you don't need — or under-buying on safety you absolutely do.
Here's the full breakdown, no fluff.
The letters in a trailer tire size designation tell you the tire's internal construction type:
These aren't ratings, grades, or quality levels. They describe how the tire's internal plies — the layers of fabric and steel that give the tire its structure — are arranged. And that arrangement affects everything about how the tire performs.
In a bias ply tire, the internal plies run diagonally across the tire from bead to bead, typically at 30-45 degree angles. Each successive ply runs in the opposite diagonal direction, creating a crisscross pattern. The sidewall and tread are essentially one unified structure.
Stiffer sidewalls. The crisscross ply pattern creates a rigid sidewall that resists flexing under load. This is good for heavy, static loads because the tire holds its shape well.
The whole tire flexes together. When a bias ply tire hits a bump or deforms under load, the tread and sidewall move as a unit. The tread can't flex independently from the sidewall.
More heat generation at speed. Because the entire tire body flexes together with every revolution, bias ply tires generate more internal friction at sustained highway speeds. More friction = more heat. And heat is what kills trailer tires.
In a radial tire, the plies run straight across the tire from bead to bead — radially, at 90 degrees to the direction of travel. On top of these radial body plies, steel belts run circumferentially (around the tire) under the tread. The tread and sidewall are structurally independent.
Flexible sidewalls with a stable tread. The radial ply/steel belt combination allows the sidewall to flex and absorb impacts while the tread stays flat on the road. This independent movement is the core advantage of radial construction.
Less heat at highway speeds. Because the sidewall and tread flex independently, radial tires generate less internal friction at sustained speeds. Less friction = less heat = longer tire life and a bigger safety margin.
Better ground contact. The stable tread belt keeps more rubber on the road surface, which improves traction and produces more even tread wear.
| Feature | Bias Ply (D) | Radial (R) |
|---|---|---|
| Internal construction | Diagonal crisscross plies | Radial plies + steel belts |
| Sidewall stiffness | Stiff — resists flexing | Flexible — absorbs impacts |
| Heat generation at speed | Higher — whole body flexes | Lower — independent flex zones |
| Ride quality | Stiffer, more road feedback | Smoother, better bump absorption |
| Tread life | Shorter — less even wear | Longer — more even tread contact |
| Load capacity | Excellent for the construction type | Excellent — often higher per ply rating |
| Puncture resistance (sidewall) | Better — stiffer sidewall resists impacts | More vulnerable to sidewall punctures |
| Puncture resistance (tread) | Lower | Better — steel belts protect tread |
| Highway suitability | Adequate for shorter trips at moderate speeds | Superior for sustained highway use |
| Cost | $$ — typically 20-35% less | $$$ — higher but longer-lasting |
| Repairability | Easier to patch | More complex due to steel belts |
| Typical speed rating | 60-65 mph max | 65 mph (some rated higher) |
This isn't a case where one is universally better. The right choice depends on how you use your trailer.
Here's the industry consensus as of now: for any trailer that regularly tows on highways, radial (R) is the recommended construction.
The reason comes down to heat. Trailer tires already run hotter than car or truck tires — they carry higher loads relative to their capacity, they're smaller (more revolutions per mile), they have no engine cooling, and they get less airflow. All of those factors stack heat.
the majority of tire blowouts trace back to underinflation and overheating. Radial tires generate less heat at highway speed. That means more thermal headroom between normal operation and failure. When your tires are already running closer to the edge than car tires, every degree of headroom matters.
Read more about tire temperature zones and what's normal in our guide on how hot a trailer tire should get.
Here's a common misconception: people think choosing radial means they can relax about load rating. Wrong. D and R describe construction type, not load capacity.
Load range (B, C, D, E) determines how much weight the tire can carry at its maximum rated inflation pressure. Both bias ply and radial tires come in various load ranges. A Load Range C radial might carry less than a Load Range D bias ply.
Always match or exceed the load range specified by your trailer manufacturer. Check your trailer's VIN plate or door jamb sticker for the original tire size and load range. If you're upgrading from D to R construction, make sure you're staying at the same (or higher) load range.
For a detailed breakdown of pressure and load capacity, see our trailer tire PSI guide.
Here's something that ties the construction type directly to measurable safety: the UTQG temperature rating.
Most radial ST trailer tires earn a temperature rating of A or B. Many bias ply ST tires come in at B or C. That rating reflects the tire's ability to resist heat at sustained speed — and it's a direct consequence of the construction differences described above.
If you're choosing between D and R for a highway trailer, the temperature rating difference alone makes a compelling case for radial. A-rated tires have measurably more thermal headroom before failure.
Don't do this. Bias ply and radial tires have fundamentally different flex characteristics. Mixing them on the same trailer creates uneven handling, uneven wear, and unpredictable behavior — especially during emergency maneuvers or crosswinds. All tires on your trailer should be the same construction type, same size, and ideally the same brand and model.
Bias ply tires aren't defective — they're a different tool for a different job. For low-speed, rough-terrain, heavy-static-load applications, bias ply construction has genuine advantages. The mistake is using them for a job they weren't designed for: sustained high-speed highway towing.
Whether you choose D or R, make sure your trailer tires carry the ST (Special Trailer) designation. ST tires are engineered specifically for trailer use — stiffer sidewalls, higher load capacity, and compounds formulated for trailer-specific heat and load conditions. Don't put LT (Light Truck) or P (Passenger) tires on a trailer unless you want to discover why the designations exist.
Radial tires can have a slightly different profile — sometimes a wider tread face or different sidewall bulge characteristics under load. When switching from D to R, check that the new tire fits your wheel well and clears fenders, suspension components, and brake hardware. This is especially important on single-axle trailers with tight fender clearance.
| Trailer Type | Typical Use | Recommended | Why |
|---|---|---|---|
| Boat trailer | Highway + launch ramp | Radial (R) | Highway speed + water exposure demands lower heat and better sealing |
| Travel trailer / RV | Highway trips, 2-8 hours | Radial (R) | Sustained speed makes radial's heat advantage critical |
| Horse trailer | Highway, precious cargo | Radial (R) | Smoother ride + thermal safety for live cargo |
| Utility trailer (highway) | Equipment hauling on highway | Radial (R) | Load + speed combination favors radial construction |
| Utility trailer (local/farm) | Short trips, dirt/gravel, low speed | Bias Ply (D) | Better sidewall toughness, lower cost, speed not a factor |
| Flatbed / equipment trailer | Heavy loads, mixed roads | Radial (R) for highway; Bias (D) for off-road | Match to the road surface and speed |
| Landscape / dump trailer | Local work, low speed | Bias Ply (D) | Toughness and value for demanding, low-speed conditions |
If your trailer sees highway speeds regularly, go radial. The lower heat generation, longer tread life, and smoother ride make it the right tool for the job. The upfront cost difference pays for itself in tire longevity and — more importantly — safety margin.
If your trailer stays on farm roads, gravel lots, and local streets below 45 mph, bias ply is a perfectly good choice that saves money and offers tougher sidewalls for rough conditions.
And regardless of which construction you choose: monitor your tires. The best tire in the world still fails if it's underinflated, overloaded, or running on a bearing that's cooking itself. most blowouts stem from preventable conditions — and prevention starts with knowing what's happening at each wheel, in real time.
The TWD Adventure monitors tire pressure, tire temperature, and axle temperature on any trailer tire — bias ply or radial. Real-time alerts on your phone mean you know the moment a tire or bearing starts running hot, before it becomes a blowout.
$395 — Made in America (Ijamsville, MD)
D stands for Diagonal, which means bias ply construction. The tire's internal plies are laid in a diagonal crisscross pattern. This creates a stiffer, more uniform structure where the sidewall and tread flex together. Bias ply tires are better suited for lower-speed, rough-terrain applications.
R stands for Radial. The internal plies run straight across (radially) from bead to bead, with steel belts running circumferentially under the tread. This allows the sidewall and tread to flex independently, which reduces heat generation at highway speeds and provides more even tread wear.
Yes, in most cases. Make sure the replacement radial tires match or exceed the original size, load range, and load capacity. Check for clearance — radial tires may have a slightly different profile under load. And don't mix: if you're switching to radial, replace all tires at once.
No. Mixing bias ply and radial tires on the same trailer creates uneven handling and unpredictable behavior. The two construction types flex differently under load and respond differently to turns, bumps, and braking. All tires on your trailer should be the same construction type.
For highway towing, yes. Radial tires run cooler (important for safety), last longer (30-50% more tread life), and ride smoother. The higher purchase price is typically offset by longer tire life. For low-speed, local-use trailers, the extra cost may not be justified.
Radial (R) is considered safer for sustained highway towing by most tire engineers and towing professionals. The primary reason is heat: radial tires generate less internal heat at highway speeds, which means more safety margin before reaching the temperature zones where tires fail. Since most blowouts are heat-related, this matters.
Most ST trailer tires — both D and R — are rated for 65 mph maximum. Some older or budget bias ply tires may be rated for 60 mph. Always check the tire's sidewall for the maximum speed rating and don't exceed it, regardless of construction type.
ST stands for Special Trailer. It indicates the tire is designed specifically for trailer use — stiffer sidewalls, higher load capacity relative to size, and rubber compounds optimized for the heat and load conditions unique to trailers. Always use ST-designated tires on your trailer, whether you choose D or R construction.
