liminfoTire Size Calculator
Free web tool: Tire Size Calculator
Tire 1 (Original)
Width
225 mm
Sidewall
101.3 mm
Diameter
634.3 mm
Circumference
1992.7 mm
Tire 2 (New)
Width
235 mm
Sidewall
94.0 mm
Diameter
645.2 mm
Circumference
2027.0 mm
Diameter Difference
+10.9 mm
Size Difference
+1.72%
Actual Speed @ 100 km/h
101.7 km/h
About Tire Size Calculator
The Tire Size Calculator lets you decode and compare any two tire sizes in the standard metric format (e.g., 225/45R17). It parses the three-part code — section width in millimetres, aspect ratio as a percentage of width, and rim diameter in inches — to compute sidewall height, overall tire diameter, and rolling circumference for each tire. Results update instantly as you type, making it easy to evaluate fitment options before you purchase.
Automotive enthusiasts, mechanics, and drivers considering a tire upgrade commonly use this tool to check whether a new tire size is compatible with their vehicle. Changing tire diameter affects not only physical clearance in the wheel arch but also the accuracy of the speedometer, which is calibrated to the original tire's rolling circumference. This calculator shows the diameter difference in millimetres and the percentage size change, plus the actual speed your car is travelling when the dashboard reads 100 km/h.
The underlying maths follows the standard formula used by tyre manufacturers: sidewall height = (section width × aspect ratio) / 100, overall diameter = rim diameter (converted from inches to mm) + 2 × sidewall height, and circumference = π × diameter. The speed correction factor is simply the ratio of the new circumference to the original circumference multiplied by the indicated speed. All computation runs entirely in your browser — no data is sent to any server.
Key Features
- Side-by-side comparison of two tire sizes with instant real-time calculation
- Displays section width, sidewall height, overall diameter, and rolling circumference for each tire
- Shows diameter difference in mm and percentage size change between the two tires
- Calculates actual vehicle speed when the speedometer reads 100 km/h after a tire swap
- Accepts standard three-part tire notation: width/aspect R rim (e.g., 205/55R16)
- Highlights plus-sizing and minus-sizing effects to help evaluate wheel fitment
- 100% client-side — no account, no upload, and no data leaves your browser
- Responsive dark-mode interface works on desktop, tablet, and mobile
Frequently Asked Questions
What does a tire size like 225/45R17 mean?
225 is the section width in millimetres (the widest point of the inflated tire). 45 is the aspect ratio — the sidewall height is 45% of 225 mm, so 101.25 mm. R means radial construction. 17 is the rim diameter in inches. The calculator uses these three numbers to derive the full tire geometry.
How is tire diameter calculated?
Sidewall height = (section width × aspect ratio) / 100. Overall diameter = (rim diameter × 25.4) + (2 × sidewall height). For 225/45R17: sidewall = 101.25 mm, diameter = 431.8 + 202.5 = 634.3 mm.
Why does changing tire size affect the speedometer?
The speedometer measures rotations of the wheel and multiplies by the original tire circumference to estimate distance per unit time. A larger tire travels further per rotation, so the speedometer under-reads — the car is going faster than displayed. A smaller tire causes the speedometer to over-read.
What is a safe percentage difference between tire sizes?
Most manufacturers recommend keeping the overall diameter within ±3% of the OEM size to avoid significant speedometer error, ABS calibration issues, and potential wheel-arch clearance problems. The calculator shows the exact percentage so you can verify compliance.
Can I use this calculator for plus-sizing?
Yes. Plus-sizing means increasing rim diameter while reducing the aspect ratio to maintain a similar overall diameter. Enter the original tire as Tire 1 and the proposed plus-size as Tire 2. The diameter difference and percentage change will confirm whether the overall rolling diameter is preserved.
What is rolling circumference and why does it matter?
Rolling circumference (π × diameter) is the distance the tire travels in one full rotation. It directly affects odometer readings, fuel consumption calculations per 100 km, ABS and traction control calibration, and gear ratio effective output. Matching circumferences between old and new tires minimises these effects.
Does the tire width affect vehicle handling?
Yes. A wider tire offers a larger contact patch, which generally improves grip in dry conditions but can reduce traction on snow or standing water. Width does not affect diameter directly, but a wider tire may require wheel-arch clearance checks.
Is there a recommended tire size comparison tool for winter tires?
This calculator is ideal for comparing summer and winter tire sets. Many drivers run narrower winter tires on a smaller rim (e.g., steel wheels). Enter both sizes to confirm the overall diameter is close enough (within ~2%) to avoid recalibrating the speedometer or triggering TPMS warnings.