Automotive
Tire Size Calculator
Compare current and proposed tire sidewall heights, overall diameters, circumference, percentage difference, and estimated actual speed at an entered indicated speed. The wheel cross-section visual draws both tire envelopes around their rims.
Current tire diameter (mm)-
New tire diameter (mm)-
Diameter difference-
Estimated actual speed-
New circumference (mm)-
Decision view
Current and proposed tire cross-sections
Current and proposed tire cross-sectionsNominal sidewalls, rims, overall diameters, and circumference-based speed effect are drawn on one proportional comparison.
Exact scenario comparisonNew tire width (mm) changes while all other entered assumptions remain constant.
| New tire width (mm) | Current tire diameter (mm) | New tire diameter (mm) | Diameter difference | Estimated actual speed | New circumference (mm) |
|---|
How to use Tire Size Calculator
- Read both complete tire sizes and confirm rim diameters.
- Compare overall diameter and speed indication as a preliminary check.
- Verify approved rim width, load index, speed rating, clearance, offset, pressure, and manufacturer fitment guidance.
Calculator guide
Understanding Tire Size Calculator
Metric tire notation encodes section width, sidewall aspect ratio, and rim diameter in mixed units. Overall diameter determines circumference and the first-order speedometer difference, but fitment requires much more than matching diameter.
Calculation method
How the calculation works
Calculate tire sidewall height, overall diameter, circumference, and the proportional speedometer effect between two metric tire sizes. Calculate two sidewalls as width times aspect ratio, add twice the sidewall height to rim diameter converted from inches, and compare the resulting diameters and circumferences.
Fitment checklist
Review the constraints the diameter comparison cannot answer
A close circumference match is only one part of safe fitment.
Worked situations
Practical examples
- 225/55R17 has an overall diameter of about 679.3 mm.
- 235/50R18 is about 692.2 mm, roughly 1.9% larger.
- At an indicated 100, the first-order modeled actual speed is about 101.9 in the same speed unit.
Better inputs
Useful tips
- Check clearance at full steering lock and suspension compression.
- Preserve or exceed required load index and speed rating.
- Recalibrate driver-assistance or pressure systems when the manufacturer requires it.
Before relying on the result
Limitations and common mistakes
- Nominal section dimensions can differ by tire model and approved rim width.
- Rim width, offset, load rating, inflation, tread wear, deflection, chains, suspension, gearing, and legal limits are excluded.
- The speed result assumes indicated speed scales directly with rolling circumference.
Reference
Key terms
- Section width
- Nominal tire width in millimeters.
- Aspect ratio
- Sidewall height as a percentage of section width.
- Overall diameter
- Rim diameter plus two nominal sidewall heights.
- Rolling circumference
- Distance traveled in one idealized tire revolution.
Important note
Calculated from the entered vehicle and operating values. Actual prices, financing terms, efficiency, maintenance, insurance, taxes, and resale outcomes can differ.
Frequently asked questions
Does a small diameter difference guarantee fitment?
No. Width, rim, offset, load capacity, and clearance can still make the combination unsuitable.
Why does actual speed rise with a larger tire?
Each wheel revolution covers more distance than the calibration assumes.
Is new circumference based on loaded radius?
No. It uses nominal geometric diameter.
Can front and rear sizes be compared for AWD?
The arithmetic is informative, but manufacturer limits on rolling circumference must control.