The numbers, explained.

1. Nominal tire geometry

For a metric tire marking such as 225/45R17, the section width is 225 mm, the aspect ratio is 45%, and the wheel diameter is 17 inches. One inch is exactly 25.4 mm.

That gives a nominal sidewall height of 101.25 mm, a diameter of 634.3 mm, and a theoretical circumference of approximately 1,992.7 mm. Revolutions per kilometer are 1,000,000 divided by the circumference in millimeters.

For a flotation marking such as 33x12.50R15, 33 inches is the nominal diameter and 12.50 inches is the nominal section width. Sidewall height is half the difference between the nominal tire and wheel diameters.

A marking describes nominal dimensions. Tire construction, the measuring rim, pressure, and the manufacturer’s measurement conditions affect actual dimensions. A nominal section width is not necessarily the full outside width of a tire with sidewall features.

2. Tire comparisons and estimated speed

Changing from 225/45R17 to 235/40R18 increases nominal diameter from 634.3 to 645.2 mm: a 10.9 mm, or approximately 1.72%, increase. The unloaded radius changes by 5.45 mm.

At an indicated 100 km/h, the estimated actual speed becomes approximately 101.72 km/h. This assumes the current tire correctly represents the speedometer’s calibration. The calculation does not account for an existing speedometer bias.

The unloaded radius change is not a measurement of ride height under load. Geometric circumference is not measured rolling circumference. Actual rolling behavior can depend on load, pressure, and tire construction.

3. Wheel position and the sign of offset

The hub mounting plane is the reference. Positive offset places that plane toward the outside of the wheel relative to its nominal centerline. With width unchanged, a higher positive offset moves the wheel inward.

Nominal wheel width is entered in inches; offset and the resulting reaches are in millimeters. These reaches use nominal rim reference planes rather than the outside edges of the lips.

Changing from 18x8 ET35 to 18x8.5 ET42 reduces nominal inner clearance by 13.35 mm and reduces outer extension by 0.65 mm. The wider wheel reaches farther inward, while the increase in offset moves it inward.

Signed values are preserved. In an extreme geometry, a reach can be negative. A negative result must not be silently converted into an absolute distance.

4. Backspacing needs physical edge geometry

Backspacing is measured from the mounting plane to the inner physical lip. It differs from nominal inner reach. The tools offer two explicit measurement models.

The first model requires the full overall width and an explicit assumption that the lips are symmetric around the nominal centerline. The second uses a measured inner flange extension. Rearrange either formula to find offset from backspacing.

No fixed flange allowance is added automatically. An overall width alone is insufficient when the lip geometry is asymmetric and unknown.

5. Combined setup and static clearances

Each tire’s wheel diameter must match its paired wheel. The simplified tire envelope is symmetric about the wheel centerline. Its inner and outer reaches use half the supplied envelope width, plus or minus offset.

The clearance measurement and reach calculation must use the same surface: tire or wheel lip. For wheel lips, the physical dimensions of both wheels are required. A missing measurement stays missing and does not become zero.

A negative projected clearance indicates contact or interference in this model. A positive result only describes a static estimate under the assumptions. It does not establish adequate clearance throughout vehicle operation.

The projection assumes the same hub position, alignment, obstacle, and measurement conditions. Steering angle, suspension travel, camber changes, tire deformation, brakes, sidewall details, pressure, and three-dimensional geometry are not inferred.

6. Units, rounding, and diagrams

Calculations use millimeters internally, with finite values and signed offset. Intermediate results are not rounded. Displayed results are rounded for readability, so adding rounded values can differ slightly from the reported total.

Metric and imperial displays use the same calculation. A metric tire marking always retains its original units, even when results are shown in inches. The diagrams illustrate nominal geometry; the text results contain the corresponding values.

Definitions and references

Manufacturer references explain the underlying measurement conventions. The clearance projection is a geometric model based on the stated assumptions.

• Yokohama — tire sizing information
• Yokohama — basic tire measurements
• Mickey Thompson — metric conversion and actual dimensions
• Mickey Thompson — backspacing and offset
• Yokohama — tire replacement considerations

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