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What does this calculator estimate?
Moving from 225/55R17 to 235/50R18 raises tire diameter from 26.74 to 27.25 inches: a 1.9% change. Your speedometer will read about 1.9% low at any speed. Compare any two sizes in the BoringToolsKit tire size comparison calculator before you buy.
- A plus-size swap that changes diameter more than about 3% can distort speedometer and odometer readings. (Source: NHTSA)
- Sidewall height in inches is width (mm) x aspect ratio / 100 / 25.4. (Source: BoringToolsKit calculation methodology)
- The overall diameter, not the wheel size, determines speedometer accuracy. (Source: BoringToolsKit calculation methodology)
How this calculator works
Formula
Diameter (in) = wheel + 2 x (sidewall), where sidewall = width x aspect ratio / 100 / 25.4; difference % = (d2 - d1) / d1 x 100.
Worked example
Replacing 225/55R17 tires (26.74 in diameter) with 235/50R18 (27.25 in) grows the diameter by 1.9%.
Assumptions to verify
- Sizes follow the standard metric tire code: width in mm, aspect ratio as a percent of width.
- Speedometer error scales with the diameter difference.
Frequently asked questions
How much diameter change is safe?
Keep the overall diameter within about 3% of the original to protect speedometer, odometer, and fitment.
Does a bigger wheel mean a bigger tire?
Not necessarily: plus-size swaps use lower-profile sidewalls to keep the diameter similar.
Cite this tool
BoringToolsKit. “Tire Size Comparison Calculator.” boringtoolskit.com/tire-size-comparison-calculator/. Free to reference in articles, syllabi, and answer posts with a link.
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