What is the Tire Size Calculator?
The Tire Size Calculator is a mechanical tool for automotive enthusiasts looking to upgrade their wheels. It compares the mathematical diameter, width, and circumference of two different tires to determine how the new rubber will affect your car's speedometer and physical fender clearance.
How to Calculate Tire Dimensions (Formulas)
Tire sidewalls display a complex metric/imperial code (e.g., 245/40R18). This calculator breaks it down automatically.
- Tread Width: The first number (245) is the width of the tire in millimeters.
- Sidewall Height Formula: Width (245mm) * Aspect Ratio percentage (40%) = 98mm sidewall.
- Total Diameter Formula: (Sidewall Height in inches * 2) + Wheel Diameter (18 inches).
Frequently Asked Questions
Why is my speedometer wrong after getting bigger tires?
Your car's computer calculates speed based on the number of rotations the axle makes. A larger tire has a larger circumference, meaning it travels physically further along the road with every single rotation. If you install massively oversized off-road tires, your speedometer might say you are driving 60 mph, but you are actually speeding at 67 mph.
What is "Plus Sizing"?
Plus Sizing is the practice of buying a larger, heavier metal wheel (like jumping from a 16-inch rim to an 18-inch rim) while simultaneously buying a tire with a much thinner sidewall. If calculated correctly, the total outer diameter of the new wheel/tire combo remains identical to the factory setup, ensuring the speedometer stays perfectly accurate.
What does the "R" stand for in a tire size?
The "R" in 245/40R18 does not stand for Rim. It stands for Radial construction, which means the internal steel cord plies run at a 90-degree angle to the direction of travel. Nearly all modern passenger car tires are radial. If you see a "D" or "B", it is a bias-ply tire, usually reserved for heavy farm equipment or classic cars.