Tire Size Calculator
Compare two tire sizes and see the difference in overall diameter, circumference and how much your speedometer reading would differ.
Tire 1
Tire 2 (comparison — optional)
How to use the Tire Size Calculator
- Enter your inputs into the Tire Size Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The tire size math
Overall diameter = wheel diameter + 2 × sidewall height · · · Sidewall = section width × aspect ratio / 100
A tire labeled "P225/65R17" means 225 mm section width, 65% aspect ratio (sidewall height = 146.25 mm), 17" wheel. Total diameter = 17×25.4 + 2×146.25 = 724 mm ≈ 28.5".
Worked example
Original: 225/65R17 → diameter 28.5". New: 245/70R17 → diameter 30.5". Difference: 2 inches taller. Speedometer at 60 mph would read about 56 mph — you're actually going 60.
Frequently asked questions
Can I go up a tire size?
Yes within limits. Most car manufacturers approve sizes within ±3% of original diameter. Bigger changes affect speedometer, gearing, handling and fender clearance.
Does tire size affect MPG?
Slightly. Larger tires raise gearing (engine spins slower at the same speed), which often improves highway MPG by 1–3% but reduces low-end torque.
How do I read tire sidewall?
Example "225/65R17 102H": 225 mm wide, 65% aspect (sidewall as % of width), R = radial, 17" wheel, 102 = load index, H = speed rating.