Tire Size Calculator Online Free Tool
Tire Size Calculator
Tire Code
Width / Aspect Ratio Construction Wheel Diameter
Tire Code Reference
Tire Dimensions
Calculation Formula:
Tire size numbers can be confusing. A tire labeled 225/55R17 packs three key measurements into one string. This calculator decodes any tire size, converts between tire sizing formats, compares two tire sizes side-by-side, and shows how a size change affects your speedometer accuracy and ground clearance.
Reading a Tire Size
The standard tire size format (P-metric) tells you: width in millimeters, aspect ratio (sidewall height as a percentage of width), construction type (R = radial), and rim diameter in inches.
225/55R17 breakdown: 225 = Section width in mm 55 = Aspect ratio (sidewall height = 225 × 0.55 = 123.75 mm) R = Radial construction 17 = Rim diameter in inches Overall diameter = Rim diameter × 25.4 + 2 × Sidewall height
225/55R17: Sidewall = 123.75 mm. Overall diameter = 17 × 25.4 + 2 × 123.75 = 431.8 + 247.5 = 679.3 mm (26.7 inches).
Tire Size and Speedometer Accuracy
Changing to a larger or smaller tire diameter affects your speedometer reading because the speedometer is calibrated to the original tire's circumference. A larger diameter tire travels more distance per revolution, making your actual speed higher than displayed. This also affects odometer accuracy and fuel economy calculations.
Actual Speed = Displayed Speed × (New Circumference / Original Circumference) Actual Speed = Displayed Speed × (New Diameter / Original Diameter)
Frequently Asked Questions
Can I use a different tire size than what came on my car?⌄
Minor variations (plus-sizing or down-sizing) are common and generally safe if done correctly. The critical constraint is that the tire must clear the wheel well, suspension, and brakes without rubbing. Keeping the overall diameter within 3% of original is generally recommended to avoid significant speedometer error and maintain ABS/traction control calibration. Consult a tire specialist for guidance.
What does the load index and speed rating mean on a tire?⌄
The load index (e.g., 91) corresponds to the maximum weight each tire can support (91 = 615 kg / 1,356 lbs). The speed rating (e.g., V) indicates the maximum sustained speed (V = 240 km/h / 149 mph). Always replace tires with equal or higher load index and speed rating to match the vehicle's specifications.
What is plus-sizing?⌄
Plus-sizing means fitting a wheel with a larger rim diameter while using a tire with a shorter sidewall (lower aspect ratio) to maintain the same overall diameter. A 17-inch rim can be swapped for an 18-inch rim by going from 225/55R17 to 225/45R18. Benefits include sharper handling and a sportier look. Trade-offs include a harsher ride and higher cost for larger rim/tire combinations.
How do I know when to replace my tires?⌄
Tires should be replaced when tread depth reaches 2/32" (1.6 mm), which is the legal minimum. Use the penny test: insert a penny head-down into the tread groove. If you can see all of Lincoln's head, the tread is too worn. The quarter test (Washington's head) at 4/32" gives more warning time. Also replace tires showing sidewall cracking, bulges, or after 6-10 years regardless of tread.