How Trailer Tire Size Affects Towing Stability, Starting With the Sidewall Numbers

Trailer tire size affects towing stability mainly through four things: how much the sidewall flexes sideways, how much load the tire can carry, how high the trailer rides, and whether the tires on an axle or tandem roll at the same circumference. A taller or softer sidewall deflects more under side loads, which can make a trailer feel less planted. An undersized or underinflated tire flexes and heats up, which can cause sway or a failure. A larger overall diameter raises the trailer slightly and changes its stance. Tire size is still only one factor. Trailer loading, tongue weight, hitch setup and speed usually matter more for sway, and the right inflation pressure often matters more than a small size change.

How Does Trailer Tire Size Affect Towing Stability?

Each part of a trailer tire size changes a different physical property that affects stability:

• Sidewall height (set by width and aspect ratio) affects how far the tire can flex sideways before the tread responds. More flex generally means lower lateral sidewall stiffness at the same construction and pressure.
• Section width affects footprint shape and, together with construction, how much cornering force the tire builds per degree of slip.
• Overall tire diameter affects ride height, trailer level relative to the hitch, and fender and tandem clearance.
• Load capacity and load range determine whether the tire carries the trailer's weight without excessive deflection and heat.
• Circumference determines how many times the tire turns per mile. Mismatched circumferences on one axle or tandem make tires scrub against each other.

To judge any of these, you first need to read the sidewall correctly.

How to Read the Numbers on a Trailer Tire Sidewall

A trailer tire sidewall carries three kinds of information: dimensions (width, rim diameter), a ratio (aspect ratio, which is not a measurement on its own), and performance ratings (load index, load range, speed symbol, maximum pressure). These values use mixed units, so it helps to read each one separately.

Example: ST205/75R15 101/97M Load Range D

The load and speed values below show how the marking works. They are an example, not the specification of every tire in this size. Always read the figures printed on your own tire.

Marking	Type	Plain-language meaning	What you can compare
ST	Service type	Special Trailer. The tire is designed for trailer use only, not for cars or trucks.	ST and P-metric (passenger) tires are rated differently and are not direct equivalents.
205	Dimension (millimetres)	Nominal section width: sidewall to sidewall at the widest point, excluding raised lettering, on a specified measuring rim.	A higher number means a wider tire on the same rim.
75	Ratio (percent)	Aspect ratio: sidewall height is 75% of section width. It has no unit of its own.	The same aspect ratio gives a taller sidewall on a wider tire.
R	Construction	Radial ply. A dash (-) indicates bias ply, and B indicates belted bias.	Construction affects sidewall flex and heat behaviour.
15	Dimension (inches)	Rim diameter the tire fits.	The tire must match the wheel exactly. A 15 in tire does not fit a 14 in or 16 in wheel.
101/97	Performance rating (code)	Load index for single and dual mounting. Index 101 = 825 kg (1,819 lb) per tire. Index 97 = 730 kg (1,609 lb) per tire when mounted as duals.	A higher index means more load capacity at the specified pressure.
M	Performance rating (code)	Speed symbol. M = 130 km/h (81 mph). L = 120 km/h (75 mph). N = 140 km/h (87 mph).	This is the tested maximum speed when properly loaded and inflated, not a recommended towing speed.
Load Range D	Performance rating (letter)	Indicates carcass strength and the pressure the tire is built to hold. Commonly, C corresponds to a 6-ply rating, D to 8-ply and E to 10-ply.	A higher load range usually allows higher pressure and more load in the same size.
Max load / max pressure	Performance rating (lb or kg; psi or kPa)	The maximum load the tire carries, stated together with the cold inflation pressure required to carry it.	Load capacity is only valid at the stated pressure.

Other markings, such as the DOT code, tread and sidewall ply composition, and treadwear-related markings, describe manufacture and construction. They do not describe size.

Older and Flotation-Style Trailer Tire Sizes

Not every trailer tire uses the metric format. Some small-trailer and utility sizes are written differently:

• Numeric bias sizes, such as 4.80-12: the first number is the approximate section width in inches, the dash means bias construction, and 12 is the rim diameter in inches. No aspect ratio is stated.
• Flotation sizes, such as 20.5x8.0-10: 20.5 in is the nominal overall diameter, 8.0 in is the nominal section width, and 10 in is the rim diameter. Diameter is given directly, so no calculation is needed.

How to Calculate Sidewall Height, Diameter and Circumference

You can calculate the nominal geometry of a metric trailer tire from its size code in four steps. Use millimetres throughout and convert at the end (1 in = 25.4 mm).

1. Sidewall height = section width × (aspect ratio ÷ 100). For ST205/75R15: 205 × 0.75 = 153.75 mm (about 6.05 in).
2. Rim diameter in millimetres = rim inches × 25.4. For a 15 in rim: 15 × 25.4 = 381 mm.
3. Overall diameter = (2 × sidewall height) + rim diameter. For ST205/75R15: 307.5 + 381 = 688.5 mm (about 27.1 in).
4. Circumference = overall diameter × π. For ST205/75R15: 688.5 × 3.1416 ≈ 2,163 mm (about 85.2 in), which is roughly 744 revolutions per mile.

Calculated Geometry of Common Trailer Tire Sizes

The table shows nominal calculated values, rounded. Actual tires differ by brand, rim width, tread depth, inflation and load.

Size	Sidewall height	Overall diameter	Circumference	Revolutions per mile
ST175/80R13	140.0 mm (5.5 in)	610 mm (24.0 in)	1,917 mm (75.5 in)	about 840
ST205/75R14	153.8 mm (6.1 in)	663 mm (26.1 in)	2,083 mm (82.0 in)	about 773
ST205/75R15	153.8 mm (6.1 in)	689 mm (27.1 in)	2,163 mm (85.2 in)	about 744
ST225/75R15	168.8 mm (6.6 in)	719 mm (28.3 in)	2,257 mm (88.9 in)	about 713
ST235/80R16	188.0 mm (7.4 in)	782 mm (30.8 in)	2,458 mm (96.8 in)	about 655
ST235/85R16	199.8 mm (7.9 in)	806 mm (31.7 in)	2,532 mm (99.7 in)	about 636

Worked Comparison: ST205/75R15 vs ST225/75R15

Moving from ST205/75R15 to ST225/75R15 keeps the same aspect ratio and rim. Because the width rises by 20 mm, the sidewall still grows:

• Section width: +20 mm (205 to 225 mm).
• Sidewall height: +15 mm (153.75 to 168.75 mm), because 75% of the extra 20 mm is added to each sidewall.
• Overall diameter: +30 mm (about +1.2 in, or +4.4%).
• Axle-to-ground height: about +15 mm (0.6 in) in nominal terms, because the axle rises by half the diameter change.
• Circumference: about +94 mm per revolution, so the larger tire turns about 31 fewer times per mile.

Comparing ST235/80R16 with ST235/85R16 shows the effect of aspect ratio alone. Width and rim are the same, but the 85-series sidewall is about 12 mm taller and the overall diameter is about 24 mm larger.

Tire Size Effect on Towing Stability: What Each Dimension Changes

Each dimension affects stability through a different mechanism. Each effect depends on the trailer, its load and the tire's construction.

Section Width and Tire Width

A wider trailer tire usually has a wider footprint and, in the same construction and load range, often builds cornering force with a little less slip. The tire's grip on the road is what resists side-to-side yaw. Width is not a guarantee of stability, though. A wider tire run underinflated, or one with a lower load range, can still be softer than a narrower tire at correct pressure.

Tire Sidewall Height and Trailer Sway

Sidewall height matters for trailer sway because the sidewall is the spring between the wheel and the tread. When a crosswind, a passing truck or a steering correction pushes the trailer sideways, the sidewall flexes before the tread fully responds. A taller sidewall, at the same construction and pressure, can flex further. This lowers lateral sidewall stiffness and can make the trailer feel delayed or wandering. Most trailer tires use high aspect ratios, typically 75 to 85, so sidewall flex is part of their normal design.

Sidewall height alone does not decide stiffness. Load range, ply construction, inflation pressure and bias versus radial design all change how a sidewall resists side loads. Bias-ply trailer tires are often described as laterally stiffer, while radial ST tires typically run cooler and keep a steadier tread footprint. In practice, a taller sidewall in a higher load range at full rated pressure can feel firmer than a shorter sidewall in a lower load range that is underinflated.

Overall Tire Diameter and Ride Height

A larger overall tire diameter raises the trailer by roughly half the diameter difference. In the ST205/75R15 to ST225/75R15 example, that is about 15 mm. This raises the center of gravity slightly. It can also tilt the trailer nose-down or nose-up relative to the hitch, which shifts weight between tandem axles and can change the effective tongue weight. A larger diameter also reduces clearance to fenders and, on tandems, to the neighbouring tire.

Load Capacity and Load Rating

A trailer tire that is too small for the load is the most direct link between size and instability. An overloaded tire deflects more, builds heat and can fail, and a sudden tread separation or blowout can trigger severe sway. Add up the trailer's loaded axle weight, divide by the number of tires, and confirm that each tire's load capacity at its stated pressure covers that share with margin. If P-metric passenger tires are used on a trailer, a commonly applied guideline is to divide their rated load capacity by 1.10, because they are not rated for trailer service.

Mixed Sizes on One Axle or Tandem

Trailer tires on the same axle or tandem should be the same size, construction and load range. Different circumferences force the smaller tire to turn more often than the larger one over the same distance. On a tandem, the tires drag and scrub, load splits unevenly, and the trailer can pull or track poorly. Even two tires with the same size code but different brands or wear levels can have measurably different actual diameters.

Why Inflation Pressure Can Matter More Than Tire Size

Inflation pressure sets how much the sidewall actually flexes, so the wrong tire pressure can cancel out any benefit of a size choice. An underinflated trailer tire has a softer, more deflected sidewall, lower lateral stiffness, more heat build-up and a reduced load capacity. The load printed on the sidewall applies only at the pressure printed next to it. Check pressure cold, before towing. Use the trailer's certification or tire placard and the tire manufacturer's load and inflation guidance. Many ST tire manufacturers recommend running at the maximum pressure shown on the sidewall.

Calculated Geometry vs Actual Tire Dimensions

The numbers calculated from a size code are nominal, so a real tire's measured dimensions will usually differ somewhat. Common reasons include:

• Rim width: section width is specified on a particular measuring rim. A narrower or wider wheel changes the actual width and sidewall shape. A commonly cited rule of thumb is about 5 mm of width change per 0.5 in of rim width.
• Manufacturer and model: tread depth and casing shape vary, so two tires of the same size code can differ in diameter.
• Load and pressure: under load, the tire's loaded radius is smaller than half its free diameter. Lower pressure makes it smaller still.
• Wear: tread wear reduces overall diameter over the tire's life.

For exact dimensions, use the tire maker's published specifications for that specific tire model. A tape measure on an inflated, unloaded tire gives a reasonable check.

Fitment Compatibility: What a Size Comparison Does Not Tell You

A dimensional comparison shows how two tire sizes differ geometrically. It does not confirm that the new size fits or is safe on your trailer. Before changing size, check these points separately:

• The wheel's diameter and approved rim width range for the tire.
• The wheel's own load rating and maximum pressure, which must equal or exceed the tire's.
• Clearance to fenders, frame and suspension through full suspension travel.
• Gap between tires on tandem or triple axles.
• The trailer's axle and suspension ratings. A tire with a higher load capacity does not raise the trailer's rated capacity.
• Matching size, construction and load range across every tire on the axle group.

How to Compare Two Trailer Tire Sizes Step by Step

1. Read both size codes and note service type (ST, P or LT), width, aspect ratio, construction and rim diameter.
2. Calculate sidewall height, overall diameter and circumference for each size using the method above.
3. Find the difference in diameter and halve it to estimate the change in ride height.
4. Compare load index or maximum load, and the pressure each load requires, against your loaded axle weight per tire.
5. Compare load range and speed symbol with how you tow.
6. Check the manufacturer's actual dimensions and approved rim widths, then confirm clearance and wheel ratings on the trailer.

What Trailer Tire Size Cannot Fix

Changing trailer tire size cannot correct sway caused by poor weight distribution, low tongue weight, excessive speed or an incorrect hitch setup. Stiffer or larger tires may change how the trailer responds, but they do not remove the underlying cause. The tire contribution to towing stability is best secured by choosing an ST tire with enough load capacity for the actual load, keeping it at the correct cold pressure, matching all tires on the axle group, and staying within the trailer's original size and fitment limits unless every compatibility point above has been checked.