Tire Size Calculator
Compare tire dimensions, overall wheel diameter, sidewall heights, and speedometer calibration offsets between original factory tires and aftermarket wheel upgrades. Enter standard tire specifications (e.g. 225/45R17 vs 245/40R18) to verify fitment and rolling circumference differences instantly.
Section width in millimetres (e.g. 205, 225, 245)
Sidewall height as a percentage of width
Wheel rim diameter in inches (e.g. 15, 17, 18, 20)
Section width for comparison or aftermarket tire
Aspect ratio percentage for comparison tire
Wheel rim diameter for comparison tire
Keep within ±3% to preserve ABS and transmission health
How to Use This Calculator
- Check your driver door jamb placard or current tire sidewall for stock size (e.g. 225/45R17).
- Input the section width, aspect ratio, and rim diameter into the Original Tire fields.
- Enter your prospective aftermarket wheel and tire upgrade dimensions into the New Tire fields.
- Verify that the overall diameter variance remains within the recommended ±3% tolerance.
- Check the speedometer correction indicator to know your true speed at 60 mph indicated.
Formula & Calculation Methodology
Sidewall height is calculated by multiplying width by aspect ratio and converting to inches. Total diameter adds rim diameter to two sidewalls. Speedometer reading scales proportionately with diameter change.
Worked Example
Upgrading from stock 225/45R17 (24.97 in) to plus-one 245/40R18 (25.72 in)
Frequently Asked Questions
Why is it crucial to keep tire diameter changes within ±3%?↓
Staying within a 3% variance ensures vehicle safety systems like Anti-lock Braking (ABS), Electronic Stability Control (ESC), and traction control function within calibrated parameters. Larger deviations can trigger warning lamps or induce false braking intervention.
How does a larger tire diameter affect acceleration and fuel economy?↓
A larger overall tire diameter effectively acts like a taller gear ratio, marginally lowering highway cruising RPM and potentially helping open-road fuel economy, while slightly reducing off-the-line throttle acceleration torque.
Does wheel rim width affect tire sidewall height and profile?↓
Yes. Mounting a tire on a wheel rim wider than the tire design measuring rim will slightly pull the sidewall outward, marginally reducing effective standing sidewall height and stiffening ride comfort.
What is the "Plus-Sizing" rule for aftermarket automotive wheels?↓
Plus-Sizing involves increasing the metal wheel rim diameter by 1 or 2 inches (e.g., Plus 1: 17 to 18 inch) while proportionally reducing tire aspect ratio to keep overall rolling diameter identical to stock.
What limitation should I check before installing wider aftermarket tires?↓
Tire calculators assess rolling dimensions but do not account for mechanical clearance against suspension struts, brake calipers, inner fender liners, or steering lock rub under full suspension compression.
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Open calculator →Source Citation & Disclaimer
Informational tool only; not licensed tax, legal, or financial advice.