Why are wheel sizes measured in inches instead of mm?

Because the automotive industry was heavily centralized in the United States during the early 20th century, wheel dimensions were standardized in inches. Even when Europe transitioned tires to metric widths in the 1960s, it was too expensive globally to redesign wheel tooling, so rims remained in inches.

What is a metric TRX wheel?

In the 1970s and 80s, Michelin attempted to introduce fully metric wheels (e.g., 390mm diameter, which is about 15.35 inches). These "TRX" wheels require special, incredibly expensive metric tires and cannot safely mount standard 15-inch or 16-inch tires.

How do I convert wheel offset from mm to inches?

Wheel offset is one of the only wheel measurements done in millimeters. To convert an offset of +44mm into inches, divide by 25.4, resulting in an offset of roughly +1.73 inches from the centerline.

Is a 200mm tire exactly 8 inches wide?

Not exactly. 8 inches equals exactly 203.2 millimeters. A 200mm tire would be slightly narrower than 8 inches.

Where can I find my wheel's dimensions stamped?

Wheel dimensions, including width, diameter, and offset, are usually cast into the metal on the backside of the wheel spokes or on the inside barrel (e.g., stamped as "18x8J ET40").

Wheel Rim Size Converter

Convert your wheel's diameter and width measurements between standard imperial inches and metric millimeters.

Quick Answer

A standard 17-inch diameter wheel converts to exactly 431.8 millimeters. If that wheel has a width of 8.0 inches, the width converts to exactly 203.2 millimeters. The conversion constant is always 25.4 mm per inch.

Enter two tire sizes (P-Metric or Flotation) to compare all specifications side-by-side. Results update live and show diameter change and estimated speedometer error.

Tire 1

Width (mm)

Aspect (%)

Rim (in)

P-Metric presets:

Tire 2

Width (mm)

Aspect (%)

Rim (in)

P-Metric presets:

Acceptable

+1.40%

Diameter difference: +0.35" (+1.4%) — acceptable for most builds

Speedometer Impact & Diameter Change1.40% error

225/45R17 Ø

24.97"

634.3mm

235/45R17 Ø

25.33"

643.3mm

Ø Difference

+0.35"

9.0mm

Speedo Error

+1.40%

Speedo reads LOW (taller tire)

Changes within about ±3% are commonly considered a safe starting range for stock vehicles.

225/45R17235/45R17— Full Comparison

Specification	Tire 1	Tire 2	Difference
Section Width (mm)	225	235	+10 (+4.4%)
Aspect Ratio (%)	45.0	45.0	—
Rim Diameter (in)	17	17	—
Sidewall Height (mm)	101.3	105.8	+4.5 (+4.4%)
Sidewall Height (in)	3.986	4.163	+0.177 (+4.4%)
Overall Diameter (mm)	634.3	643.3	+9.0 (+1.4%)
Overall Diameter (in)	24.97	25.33	+0.35 (+1.4%)
Circumference (in)	78.45	79.57	+1.11 (+1.4%)
Revs per Mile	808	796	-11 (-1.4%)
Revs per km	502	495	-7 (-1.4%)

Difference = Tire 2 − Tire 1 with % change relative to Tire 1. Values are geometric estimates based on nominal sizes.

Common Wheel Sizes (Inches to Millimeters)

Dimension Type	Imperial Size (Inches)	Metric Equivalent (mm)
Diameter	15 Inches	381.0 mm
Diameter	17 Inches	431.8 mm
Diameter	20 Inches	508.0 mm
Width	7.5 Inches	190.5 mm
Width	8.0 Inches	203.2 mm
Width	9.5 Inches	241.3 mm

Formula

Millimeters = Inches × 25.4 | Inches = Millimeters / 25.4

25.4 = exact mathematical constant between 1 inch and 1 millimeter
Example: 8.5 inches × 25.4 = 215.9 mm rim width
Offset is always in millimeters — divide by 25.4 to get inches
Tire tread width is metric but rim diameter stays in inches — a mixed standard

Frequently Asked Questions

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