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01 / TIRE TOOLS

Tire Size Comparison

Current vs. new tire geometry.

GEOMETRY CHECK

Your setup

A / Current
Metric: 225/45R17 · Flotation: 33x12.50R15
Enter separate dimensions
B / New
Metric: 225/45R17 · Flotation: 33x12.50R15
Enter separate dimensions

What changes

NOMINAL GEOMETRY

Example calculation. Replace these values with your setup.

Diameter change+1.72%

The new tire has a larger nominal diameter.

Nominal tire geometry

Change in unloaded radius
+5.5 mm
Estimated speed change
+1.72%

See the tire measurements

Diameter

Current / A
634.3 mm
New / B
645.2 mm
Change
+10.9 mm

Across the whole tire, from one outside edge to the other. This is the unloaded diameter.

ACurrent / A

Current / A: Diameter, 634.3 mmView from the front. The arrow spans the free tire diameter. Current and new configurations share one scale.Diameter634.3 mmSeen from the front

BNew / B

New / B: Diameter, 645.2 mmView from the front. The arrow spans the free tire diameter. Current and new configurations share one scale.Diameter645.2 mmSeen from the front
Orange arrows show the selected measurement. Change your inputs to update the drawing. Values describe static geometry.
Geometry results
MeasurementCurrentNewChange
634.3 mm645.2 mm+10.9 mm
225.0 mm235.0 mm+10.0 mm
101.3 mm94.0 mm-7.3 mm
Unloaded radius317.2 mm322.6 mm+5.5 mm
Geometric circumference1,992.7 mm2,027.0 mm+34.2 mm
Estimated revolutions / km501.8493.4-8.5
Estimated revolutions / mile807.6794.0-13.6
Change in unloaded radius——+5.5 mm
Estimated speed change——+1.72%
Assumptions & limits
  • Markings describe nominal geometry. Actual tire dimensions vary by tire model, measuring rim, pressure, and wear.
  • Unloaded radius and geometric circumference do not measure loaded ride height or actual rolling circumference.
  • Load and speed markings and approved tire-to-rim width combinations are not evaluated by these geometry calculations.
  • The estimated speed change assumes accurate calibration with the current tires.
CONTINUE WITH THIS SETUPEstimate speed change Add wheels to this comparison

Compare tire diameter and width

See how a replacement tire changes nominal diameter, section width, sidewall height and unloaded radius. Compare dimensions before assessing a complete setup.

Example · 225/45R17 → 235/40R18

Nominal diameter increases from 634.3 mm to 645.2 mm: a change of 10.9 mm, or 1.72%. The unloaded radius increases by 5.45 mm.

The unloaded radius change is not a measured increase in ride height under load.

Tire comparison questions

Can I compare tires for different wheel diameters?

Yes. For example, a 17-inch tire and an 18-inch tire can be compared geometrically. Each tire still requires a wheel with the matching rim diameter; a tire comparison does not establish wheel compatibility.

Does a small diameter difference mean the new tire will fit?

A diameter percentage alone does not establish fitment. Actual tire dimensions, approved rim widths, load and speed ratings, clearances and vehicle requirements also need checking.

How do I check the effect on speed?

Use the speedometer error calculator with the current tire as the calibration reference and the same replacement size. It estimates the speed ratio from nominal circumference rather than measuring actual speed.

Methodology & limits

How this tool works

Diameter change = new diameter − current diameter. Percentage change = 100 × (new / current − 1). Change in unloaded radius is half the diameter change.