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Generated on: July 20, 2026 at 09:33 PM

Tire Size Comparison Calculator

Inputs

Width (mm)225
Aspect ratio (%)45
Rim diameter (inches)18
Width (mm)245
Aspect ratio (%)40
Rim diameter (inches)19
Indicated speed17 km/h

Calculator assumes the manufacturer-spec outer diameter; real-world loaded diameter is 2–4% smaller due to sidewall flex. Always cross-check with manufacturer load and clearance data before fitting.

Generated with OneCalc · https://onecalc.app/en/lifestyle/tire-size-comparison

Lifestyle

Tire Size Comparison Calculator

Compare two tire sizes: outer diameter, difference in mm and %, ride-height change, actual road speed, and speedometer error.

Inputs

Original tire

100 – 400
25 – 85
10 – 24

Replacement tire

100 – 400
25 – 85
10 – 24

Speed check

0 – 300 km/h

Results

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Tire Size Comparison Explained

Swapping tires is one of the most common modifications drivers make — for aesthetics, performance, or simply replacing a discontinued size. The critical question is how much the outer diameter changes, because that single number determines your speedometer accuracy, ABS calibration, and whether the tire clears your fenders.

How to Read a Tire Size

A tire spec like 225/45R18 encodes three measurements:

  • 225 — section width in millimetres
  • 45 — aspect ratio: the sidewall height is 45% of 225 mm = 101.25 mm
  • R18 — radial construction, rim diameter 18 inches

The outer diameter formula is:

D=Rin×25.4+2×W×A100D = R_{\text{in}} \times 25.4 + 2 \times W \times \frac{A}{100} D225/45R18=18×25.4+2×225×0.45=457.2+202.5=659.7 mmD_{225/45R18} = 18 \times 25.4 + 2 \times 225 \times 0.45 = 457.2 + 202.5 = 659.7 \text{ mm}

Speedometer Error and Ride Height

When you fit a taller tire, each wheel rotation covers more ground. The speedometer — calibrated for the original tire circumference — reads low.

Error%=D2−D1D1×100\text{Error\%} = \frac{D_2 - D_1}{D_1} \times 100 Actual speed=Vindicated×D2D1\text{Actual speed} = V_{\text{indicated}} \times \frac{D_2}{D_1}

The ride-height change is simply half the diameter difference (one tyre radius on each side):

Δh=D2−D12\Delta h = \frac{D_2 - D_1}{2}

Quick Reference: Common Plus-Sizing Pairs

OriginalReplacementΔ diameterError
205/55R16215/50R17+14.9 mm+2.36%
225/45R18245/40R19+18.9 mm+2.86%
235/55R17255/50R18+21.9 mm+3.17%

All examples are within or near the ±3% safe tolerance.

Frequently Asked Questions (FAQ)

How do I read a tire size like 225/45R18?

225 is the section width in mm. 45 is the aspect ratio — the sidewall is 45% of 225 mm = 101 mm tall. R means radial construction. 18 is the rim diameter in inches. Outer diameter = 18 × 25.4 + 2 × 101 ≈ 659 mm.

Why does a different tire size change my speedometer?

A speedometer measures wheel rotations and multiplies by the calibrated tire circumference. A taller tire covers more ground per rotation, so the car actually goes faster than the speedometer says. A shorter tire is the reverse.

How much diameter change is safe?

±3% is the conventional tolerance — it keeps speedometer error inside legal limits in most jurisdictions and avoids ABS/traction-control miscalibration. Beyond ±3%, also check fender clearance, suspension travel, and load index.

What is "plus sizing"?

Plus-one (or plus-two) sizing fits a larger-rim, lower-profile tire while keeping outer diameter nearly identical. The goal is sharper steering response and a more aggressive look without affecting speedometer, gearing, or clearance.

Disclaimer

Calculator assumes the manufacturer-spec outer diameter; real-world loaded diameter is 2–4% smaller due to sidewall flex. Always cross-check with manufacturer load and clearance data before fitting.

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