Tire Size Comparison Calculator — Compare Tire & Tyre Sizes

In 225/65R17, 225 is nominal section width in millimeters, 65 is the sidewall height as a percentage of that width, R denotes radial construction, and 17 is the rim diameter in inches. The aspect ratio is a percentage, not a sidewall height in millimeters.

This calculator supports common metric radial codes with an optional P or LT prefix, including lowercase input and spaces. It calculates geometry from width, ratio and rim diameter. Prefixes do not establish load capacity or make sizes interchangeable.

The supported range is 100–455 mm whole-number width, 20–100% whole-number aspect ratio, and 10–30 inch rims in whole or half-inch increments. Flotation codes such as 33x12.50R15, non-radial construction, load indices and speed-rating suffixes are outside this parser. Enter just the supported size portion.

1. Enter your original nominal tire code as Size A and the proposed size as Size B.
2. Edit either code or the individual width, aspect ratio and rim diameter fields. Valid entries update both representations.
3. Read the diameter change, idealized ride-height difference, scale diagram and numerical comparison chart.
4. Enter an indicated speed to estimate the road-speed change assuming calibration to the original tire.

The examples 225/65R17 and 235/60R18 have nominal diameters of 28.52 and 29.10 inches. Their difference is about +2.06%. Swapping them changes the percentage denominator to the other tire, so the reversed percentage is not exactly −2.06%.

Copy Summary, Export CSV and Print Result use the same computed comparison. Printed values are rounded for readability; the CSV retains the underlying numerical values.

For width W in millimeters, aspect ratio A as a percentage number, and rim diameter R in inches:

• Sidewall height = W × A ÷ 100
• Rim diameter in mm = R × 25.4
• Overall diameter in mm = R × 25.4 + 2 × W × A ÷ 100
• Overall diameter in inches = diameter in mm ÷ 25.4

For 225/65R17, sidewall height is 146.25 mm and rim diameter is 431.8 mm. The nominal overall diameter is 431.8 + 2 × 146.25 = 724.3 mm, or about 28.52 inches. For 205/55R16 it is about 24.88 inches.

Diameter change = (new diameter − original diameter) ÷ original diameter × 100. The idealized ride-height change is half the diameter difference. This describes unloaded geometry rather than a measured change on a particular vehicle.

Sidewall height depends on both section width and aspect ratio. A 65% profile means the sidewall is 65% of the specified width, so the same ratio produces different sidewalls on different widths.

Nominal circumference is π × diameter. Approximate revolutions per mile equal 1,609,344 ÷ circumference in mm; revolutions per kilometer equal 1,000,000 ÷ circumference in mm. A larger nominal circumference means fewer rotations over the same road distance.

Every conversion in the chart uses the unrounded geometry. Millimeters per inch are exactly 25.4. The displayed circumference and revolutions are theoretical estimates: measured rolling circumference can differ because of inflation, load, construction and tread wear.

When speed is inferred from wheel rotation and the calibration is unchanged, a different rolling circumference changes the road distance associated with each rotation. This calculator assumes the original calibration corresponds to Size A’s nominal circumference.

Estimated actual speed = indicated speed × new diameter ÷ original diameter. For the supplied example, an indicated 60 mph becomes about 61.23 mph in the idealized model. The same ratio applies to km/h when input and output use that same unit.

The speed correction is estimated actual speed minus indicated speed. Actual distance is likewise approximately indicated distance multiplied by the new/original diameter ratio. The reciprocal, shown as indicated/actual distance, describes how much the odometer would indicate per unit actually traveled under that assumption.

These are estimates, not verified vehicle speed or odometer calibration. Real instrumentation may already have an offset, and changes to calibration alter the result. See Discount Tire’s speedometer accuracy guidance for context.

Tire and tyre refer to the same component. Tire is the usual U.S. spelling, while tyre is common in British English and many other regions. A tyre dimensions calculator uses the same millimeter, percentage and inch measurements.

Spelling does not change the formulas, but regional markings and tire standards can carry additional requirements beyond the three dimensions entered here.

A roughly ±3% nominal-diameter comparison is sometimes used as an initial screening heuristic. It is not a vehicle-specific limit, manufacturer approval or guarantee of safe fitment. This calculator never marks a size safe or compatible based on that percentage.

Even an identical overall diameter can hide a different width, rim diameter or required rim-width range. Brake clearance, steering clearance, suspension travel, load capacity and other requirements need separate checks.

Use the numerical difference to understand the geometry, then verify the proposed tire and wheel combination with the vehicle and tire manufacturers or a qualified professional.

The rim diameter in the tire code must match the wheel diameter, but this alone cannot establish fitment. An approved wheel-width range, clearance and the vehicle’s specified tire requirements also matter. Similar outer dimensions do not make different wheel diameters interchangeable.

CalcVibes calculates nominal dimensions only. It does not determine rim width, wheel offset, bolt pattern, brake fitment, TPMS calibration or road approval. Before buying or mounting a different size, follow vehicle- and tire-manufacturer fitment guidance or obtain qualified professional verification.

Michelin explains where to find your vehicle’s tire-size specifications, including the vehicle placard and owner’s manual.

A tire’s full sidewall marking contains information beyond width, aspect ratio and rim diameter. Load index and speed rating are not inferred by this tool, and a P or LT prefix alone is insufficient to compare service capabilities.

Check the actual tire specification, permitted wheel-width range and vehicle requirements together. A geometric match cannot confirm equal load capacity, braking performance or suitability for the intended use. Manufacturer data describes the specific tire; this calculator describes a nominal size code.