Tire Size Calculator
Compare two tire sizes to see the diameter and speedometer difference.
Try an example
+1.81%
About this calculator
How This Tire Size Calculator Works
Enter two tire sizes (width, aspect ratio, and rim diameter for each), and the calculator returns the difference in overall diameter — which affects speedometer accuracy and gearing.
Worked Example
Going from a 225/50R17 to a 235/45R18 increases the overall tire diameter by about 1.81% — a small enough change that most speedometers won't need recalibration.
The Formula
Diameter = Rim size + 2 × (Width × Aspect ratio ÷ 100) ÷ 25.4. Tire sidewall height is the width multiplied by the aspect ratio percentage; doubling it (for both the top and bottom of the wheel) and adding the rim diameter gives the tire's total outer diameter, converted from millimeters to inches.
Why This Matters
Your speedometer calculates speed based on wheel rotation speed, calibrated to your vehicle's original tire diameter. A larger tire covers more ground per rotation, so the speedometer under-reads your actual speed (you're going faster than it shows); a smaller tire makes it over-read. Large diameter changes can also affect gearing, fuel economy, ABS and traction control calibration, and ground clearance.
A Common Guideline
Many tire professionals suggest staying within about 3% of the original overall diameter when changing tire sizes, to avoid meaningfully affecting speedometer accuracy and vehicle systems that rely on wheel-speed sensors — though manufacturer-specific tolerances can vary.
Worked example
Going from a 225/50R17 to a 235/45R18 increases the overall tire diameter by about 1.81% — a small enough difference that most speedometers won't need recalibration.
Frequently asked questions
Why does tire size affect my speedometer?
Your speedometer calculates speed from wheel rotation speed, calibrated to your original tire's diameter. A larger-diameter tire covers more ground per rotation, so the speedometer under-reads your actual speed; a smaller tire makes it over-read.
How much of a diameter difference is considered safe?
A common rule of thumb is staying within about 3% of the original diameter to avoid meaningfully affecting speedometer accuracy, gearing, and ABS/traction control systems — though always check your vehicle manufacturer's specific tolerances.