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Understand the dimensions

Clear explanations for reading tire and wheel calculations.

Wheel geometry comparisonNominal dimensions. Not a vehicle fitment check. Wheel geometry comparison Mounting face (x = 0)Original · 8 in · ET 45 mmNew · 8.5 in · ET 35 mm-125-100-75-50-2502550Vehicle sideOutside0 mm3.65 mmNominal inner space change16.35 mmOuter position change Nominal dimensions. Not a vehicle fitment check.

Wheel geometry

Understanding wheel offset signs

Learn what positive, zero and negative wheel offset mean, and follow ET changes from a fixed mounting plane.

How to read a metric tire size

Read width, aspect ratio and rim diameter in 225/45R18, and distinguish those dimensions from load and speed markings.

Tire diameter versus wheel diameter

Understand why an 18-inch wheel is not an 18-inch tire and how sidewall height changes a tire’s overall diameter.

Understanding wheel offset signs

Learn what positive, zero and negative wheel offset mean, and follow ET changes from a fixed mounting plane.

Nominal tire size versus measured dimensions

Separate dimensions calculated from a tire marking from manufacturer measurements and measurements of a specific mounted tire.

Why rolling circumference can differ

Understand the distinction between π times nominal diameter and distance traveled per tire revolution under real conditions.

What changes when tire diameter changes

Follow a nominal diameter change through radius, circumference, revolution count and the theoretical speed comparison.

Comparing two sizes without guessing fitment

Use tire geometry to describe a change while keeping vehicle fitment, ratings and clearance outside the calculation’s claims.

Reading inner and outer wheel position

Separate inward reach, nominal inner space and outward position when comparing wheel widths and offsets.

Millimeters, inches and tire dimensions

Convert tire and wheel measurements consistently, preserve precision and distinguish display units from the size marking.

How the speedometer comparison is calculated

Follow the circumference ratio used by TireSpan’s theoretical speed rows and see why the original size and calibration assumption matter.

Choosing a print-friendly comparison report

Choose A4 or Letter, keep the original and new sizes identifiable, and use a report that preserves the calculation’s assumptions.

Saving a tire comparison locally

Understand browser-local project storage, temporary mode and JSON backups before relying on a saved comparison.

See how two tire sizes compare.

Compare diameter, sidewall height and the theoretical speed change.

Compare sizes

Read the result with its assumptions

These guides explain what a dimension means, how the calculation works and where the model stops. Examples use the same formulas as the tools, so you can follow a number from the article into a comparison.

Choose a starting point

Start with metric tire markings if the notation is unfamiliar. Continue to diameter and circumference for tire comparisons, or offset and inner/outer position for wheels. The report and local-storage guides cover keeping the calculation for later.

Keep exploring

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