Tire Size Calculator - Calculator See


Tire Size Calculator

Compare tire diameters, sidewall heights, circumference, and speedometer speed variance when changing wheel and tire sizes.

Calculator Inputs

mm
Section width (e.g. 225).
%
Sidewall ratio (e.g. 50).
in
Wheel rim size.
mm
New section width (e.g. 245).
%
New sidewall ratio (e.g. 45).
in
New rim size.

Calculated Results

Diameter Difference
+0.8 Inches (+3.0%)
Original Tire Diameter (225/50R17) 25.9 inches (657 mm)
New Tire Diameter (245/45R18) 26.7 inches (678 mm)
Speedometer at 60 mph Shows 58.2 mph (Slow by 1.8 mph)
Values calculate live as you adjust inputs.

About Tire Size Calculator

Compares outer tire diameter, sidewall thickness, revs per mile, and speedometer calibration variance when upgrading wheels or tires.

What is Tire Size Calculator?

Compares outer tire diameter, sidewall thickness, revs per mile, and speedometer calibration variance when upgrading wheels or tires.

How to Use This Calculator

  1. Enter original tire code (Width/Ratio/Rim).
  2. Enter new tire code.
  3. Review height differences and speedometer accuracy.

The Mathematical Formula & Variables

Sidewall Height = Width * (Ratio / 100) / 25.4 inches; Overall Diameter = Rim + 2 * Sidewall

Step-by-Step Worked Example

Scenario: Switching from 225/50R17 to 245/45R18.

  1. Tire 1 diameter = 17 + 2 * (225 * 0.50 / 25.4) = 25.9 inches.
  2. Tire 2 diameter = 18 + 2 * (245 * 0.45 / 25.4) = 26.7 inches.
  3. Difference = +0.8 inches (+3.0%). At 60 mph indicated, actual speed is 61.8 mph.

Result: +0.8 in Difference (Speedo off by 3%)

Frequently Asked Questions

What is the safe tire diameter variance limit?

Automotive engineers recommend keeping overall tire diameter changes within ±3% to prevent braking and ABS/traction control errors.