Dual Tire Diameter Matching: Reading Sidewall Numbers and Tread Depth Differences

Short answer: Two tires mounted side by side as a dual pair should have very nearly the same overall diameter, because they share one hub and must turn at the same speed. A widely published guideline from truck tire makers is a maximum mismatch of 1/4 inch (about 6 mm) in diameter, or 3/4 inch (about 19 mm) in circumference. The sidewall size tells you the nominal diameter. Tread depth is the most common reason two same-size tires end up mismatched, and every 1/32 inch of tread depth difference becomes 2/32 inch of diameter difference. In practice, a tread depth difference of about 4/32 inch (about 3.2 mm) between same-model tires already reaches the 1/4-inch diameter limit.

What Is Dual Tire Diameter Matching?

Dual tire diameter matching means pairing the inner and outer tire of a dual assembly so that their overall diameters and rolling circumferences are within a small tolerance. Both tires are bolted to the same hub, so they complete the same number of revolutions per mile. If one tire is larger, it has to cover more ground per revolution than the smaller one. The pair resolves that conflict in two ways: the larger tire carries more of the load, and the smaller tire is dragged and scrubbed along the road.

Matching applies to heavy truck and trailer duals and to dual-rear-wheel light trucks. In both cases the same principle holds. Matched dual tires roll together, while a dual tire diameter mismatch makes one tire do more work than the other.

Which Sidewall Numbers Matter for Dual Tire Matching?

The size designation is the key sidewall marking for matching, because it defines the tire's nominal geometry. The load and pressure markings tell you how much each tire may carry in dual service. Other markings identify the tire but say nothing about its diameter.

The size designation: dimensions and a ratio

A metric truck size such as 295/75R22.5 is read as follows:

• 295 is the nominal section width in millimetres, measured from sidewall to sidewall on a specified rim.
• 75 is the aspect ratio. This is not a length. It means the sidewall height is 75% of the section width.
• R indicates radial construction. It is a construction code, not a dimension.
• 22.5 is the rim diameter in inches.

The size code mixes units, with width in millimetres and rim diameter in inches. You need to convert one of them before you calculate. Light truck sizes such as LT235/80R17 follow the same logic. Some truck sizes, such as 11R22.5, give the width in inches and do not print an aspect ratio. For those sizes, the overall diameter is best taken from the manufacturer's published data rather than calculated from the code.

Load index and speed symbol: performance ratings, not dimensions

Truck and many LT tires carry a double load index, for example 144/141L. The first number is the load index for single fitment. The second number is the load index for dual fitment. On the standard load index table, 144 corresponds to 2,800 kg and 141 to 2,575 kg per tire. The letter L is the speed symbol for 120 km/h. These are performance ratings. They help you compare load capacity, but they do not describe size.

The dual rating is lower than the single rating for the same tire. That reduction reflects the fact that dual tires do not share load perfectly, so every tire in a pair should be able to carry its rated dual load.

Maximum load and pressure statements

Many truck tires also print separate maximum load figures for single and dual use, each at a stated cold inflation pressure in kPa and psi. These figures are limits. They do not tell you the tire's diameter. They do matter for matching, however, because inflation pressure affects the loaded radius. Dual partners are normally set to the same cold pressure.

Markings that do not indicate diameter

Several markings identify the tire but tell you nothing about its diameter:

• The load range or ply rating letter, such as G or H.
• The DOT code, including the manufacturing week and year.
• The tread wear indicator marks.
• The brand and model name.

Two tires with identical size markings can still differ in actual diameter. Brand, tread pattern, original tread depth and wear all change the real dimensions.

Marking example	Type	Plain meaning	Use in dual matching
295	Dimension (mm)	Nominal section width	Sets sidewall height together with the aspect ratio
75	Ratio (%)	Sidewall height as a percentage of width	Used to calculate sidewall height
R	Construction code	Radial tire	Dual partners are normally the same construction
22.5	Dimension (inches)	Rim diameter	Base of the overall diameter calculation
144/141	Performance rating	Load index, single/dual	Gives load capacity per tire in dual use, not size
L	Performance rating	Speed symbol (120 km/h)	No effect on diameter
Max load dual at stated psi/kPa	Limit	Maximum load per tire in dual fitment at a cold pressure	Indicates the pressure basis, since pressure affects loaded radius

How Do Width, Aspect Ratio and Rim Diameter Set Overall Diameter and Circumference?

The overall tire diameter equals the rim diameter plus two sidewall heights. Sidewall height equals section width multiplied by the aspect ratio. Circumference equals the diameter multiplied by π (about 3.1416). Because there is a sidewall above and below the rim, any change in sidewall height counts twice in the overall diameter.

Formula

• Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100
• Overall diameter (mm) = rim diameter (in) × 25.4 + 2 × sidewall height (mm)
• Circumference (mm) = overall diameter (mm) × π
• Calculated revolutions per mile = 63,360 inches ÷ circumference (inches)

Worked example: 295/75R22.5

1. Sidewall height: 295 × 0.75 = 221.25 mm (about 8.71 in).
2. Rim diameter: 22.5 × 25.4 = 571.5 mm.
3. Overall diameter: 571.5 + 2 × 221.25 = 1,014 mm (about 39.9 in).
4. Circumference: 1,014 × π ≈ 3,186 mm (about 125.4 in).
5. Calculated revolutions per mile: 63,360 ÷ 125.4 ≈ 505.

Comparing two sizes

A 275/80R22.5 has a sidewall of 275 × 0.80 = 220 mm. That gives a calculated overall diameter of 571.5 + 440 = 1,011.5 mm (about 39.8 in) and a circumference of about 3,178 mm (about 125.1 in). On paper, the gap to a 295/75R22.5 is only about 2.5 mm in diameter and about 8 mm in circumference.

The comparison shows that a narrower tire with a higher aspect ratio can reach almost the same diameter as a wider, lower-profile one. It does not show that the two sizes belong on the same dual assembly. Their widths, real dimensions and load ratings still differ. Dual partners are generally expected to be the same size and construction.

Calculated Geometry vs Actual Tire Dimensions

A diameter calculated from the size code is a nominal figure. It is not a measurement of a specific tire. Actual overall diameter and rolling circumference depend on several factors:

• The tire maker's design and the original tread depth.
• The width of the rim the tire is mounted on.
• Inflation pressure.
• Load, because a loaded tire deflects and its effective rolling radius is smaller than its free radius.
• Wear.

Manufacturers' data books list measured or design values for overall diameter, loaded radius and revolutions per mile. Those figures are more reliable than a calculation for a specific model. For an actual pair of tires on a vehicle, only a measurement shows the real mismatch.

Neither the calculation nor matching numbers confirm vehicle fitment. Dual spacing, wheel offset, clearance to the body and brakes, and load capacity have to meet the vehicle and wheel manufacturer's specifications as a separate requirement.

How Does a Dual Tire Tread Depth Difference Change Diameter?

A dual tire tread depth difference changes overall diameter by twice the difference, because tread is lost at both the top and the bottom of the tire's diameter. The circumference changes by that diameter difference multiplied by π. This conversion assumes the two tires were the same size and model when new, so the tread depth difference is the only source of mismatch.

Tread depth is commonly measured in 32nds of an inch. One 32nd of an inch equals about 0.79 mm.

Tread depth difference	Diameter difference	Circumference difference	Against a 1/4 in (6 mm) diameter guideline
1/32 in (0.8 mm)	1/16 in (1.6 mm)	about 0.20 in (5 mm)	Within
2/32 in (1.6 mm)	1/8 in (3.2 mm)	about 0.39 in (10 mm)	Within
3/32 in (2.4 mm)	3/16 in (4.8 mm)	about 0.59 in (15 mm)	Within
4/32 in (3.2 mm)	1/4 in (6.4 mm)	about 0.79 in (20 mm)	At the limit
6/32 in (4.8 mm)	3/8 in (9.5 mm)	about 1.18 in (30 mm)	Exceeds
8/32 in (6.4 mm)	1/2 in (12.7 mm)	about 1.57 in (40 mm)	Exceeds

Tread depth alone is a reliable stand-in only when both tires are the same make, model and size. Different brands of the same size can start with different new diameters and different original tread depths. Two tires can have equal remaining tread and still be mismatched, or have unequal tread and still be close in diameter. For mixed brands or retreads, measure diameter or circumference directly.

What Is the Dual Tire Circumference Tolerance?

The dual tire circumference tolerance most often published by truck tire manufacturers is 3/4 inch (about 19 mm) of rolling circumference. The equivalent diameter tolerance is 1/4 inch (about 6 mm). The two figures describe the same limit, because 1/4 inch of diameter multiplied by π is about 0.79 inch of circumference.

Some guidance also expresses the limit as a maximum tread depth difference, typically around 4/32 inch, between same-model tires. Tolerances can differ between tire manufacturers, fleets and regions. Where a manufacturer or fleet specifies its own limit, that limit takes precedence over a general rule.

What Happens When Dual Tires Are Mismatched?

A mismatched dual pair causes unequal load sharing and scrubbing. The larger tire supports more of the axle load than its share, so it deflects more and runs hotter. The smaller tire carries less load but is forced to slip against the road surface to keep pace with its partner. Common consequences include:

• Faster and irregular wear on both tires.
• Higher operating temperature in the larger tire.
• A risk that the larger tire is overloaded relative to its dual rating.

Worked example (calculated geometry): Take two 295/75R22.5 tires of the same model, one with 6/32 inch less tread than the other.

1. Diameter difference: 2 × 6/32 = 3/8 inch.
2. Circumference difference: 3/8 × π ≈ 1.18 inches per revolution.
3. Revolutions per mile at the calculated 125.4-inch circumference: about 505.
4. Slip per mile: 505 × 1.18 ≈ 596 inches, or roughly 50 feet.

In other words, the smaller tire would have to slip about 50 feet for every mile travelled if no other compliance in the tires absorbed the difference. Real tires flex and redistribute load, so this figure illustrates the scale of the conflict rather than predicting a measured slip.

How to Measure Dual Tire Mismatch

Measuring dual tire mismatch means comparing actual tread depth, circumference or diameter under the same conditions, rather than relying on the sidewall size.

Before measuring

• Check both tires cold, before driving.
• Inflate both tires to the same specified pressure.
• Clean the tread surfaces.
• Use the same tool and the same method on both tires.

Tread depth measurement (same-model tires)

1. Use a tread depth gauge in a major groove, not on a tread wear indicator bar.
2. Take readings in several grooves and at several points around each tire.
3. Average the readings for each tire.
4. Subtract the smaller average from the larger one to find the tread depth difference.
5. Double the difference to estimate the diameter difference, and compare it with the applicable tolerance.

Circumference measurement

1. Wrap a flexible steel tape around the centre of the tread, keeping it straight and seated on the tread surface.
2. Read the circumference and repeat at least once to confirm the reading.
3. Measure the partner tire the same way.
4. Compare the difference with the circumference tolerance, commonly 3/4 inch (19 mm).
5. To convert a circumference difference to a diameter difference, divide it by π.

A tape circumference measures the unloaded tire. It is the most direct comparison between two tires, especially tires of different brands or retreads.

Diameter measurement on the vehicle

Tire matching gauges, calipers or a straightedge laid across the tread of the paired tires can show which tire stands taller. They are useful for quick checks on mounted duals. Readings taken while the tires are loaded reflect deflection as well as diameter, so treat them as a comparison between partners rather than as an absolute size.

What Matched Numbers Do Not Tell You

Matching sidewall sizes and a measured difference within tolerance show that two tires are geometrically compatible as dual partners. They do not establish several other things:

• That the size is approved for the vehicle.
• That the load index and dual load rating are sufficient for the axle load.
• That dual spacing and wheel offset are correct.
• That the tires are in serviceable condition.

A calculated diameter is a nominal reference, a tape measurement describes one tire at one pressure, and a load index is a rating rather than a dimension. Reading each marking for what it actually represents is what makes dual tire matching reliable.