Compare two tire sizes side by side. See how diameter, speedometer accuracy, and ground clearance change when you switch to a different tire size.
When your new tires are fitted, your speedometer will display a slightly different speed than your actual speed.
| Actual Speed | Speedometer Reads | Difference |
|---|
These tire sizes are compatible.
Every tire has a standardized size code printed on its sidewall. Understanding this code is essential before using the calculator. Here is what each part of a typical tire size like 205/55R16 means:
The first number is the tire's section width measured in millimeters from sidewall to sidewall. A 205 tire is 205 mm (about 8.1 inches) wide. Wider tires generally provide better grip but may increase rolling resistance and fuel consumption.
The aspect ratio is the sidewall height expressed as a percentage of the section width. For a 205/55 tire, the sidewall is 55% of 205 mm, which equals 112.75 mm. Lower aspect ratios (e.g., 35 or 40) mean a shorter sidewall, giving sportier handling but a firmer ride. Higher ratios (e.g., 70 or 80) provide more cushion and comfort.
The letter indicates how the tire is built. "R" stands for Radial, which is the standard construction for nearly all passenger tires today. Radial tires have cord plies arranged at 90 degrees to the direction of travel, offering better fuel efficiency and tread life.
The last number is the diameter of the wheel (rim) that the tire fits, measured in inches. This number must match your wheel exactly. You cannot mount a 16-inch tire on a 17-inch wheel.
Most manufacturers recommend staying within 3% of the original tire's overall diameter. Changes within 1.5% are generally considered fully compatible and should not cause any drivability or safety issues. Between 1.5% and 3%, you may notice minor speedometer inaccuracy and slight changes in ride height. Exceeding 3% can affect ABS, traction control, stability systems, and may even rub against fenders or suspension components.
Yes. Your vehicle's speedometer is calibrated for the original tire size. A larger tire covers more ground per revolution, meaning your actual speed is higher than what the speedometer displays. Conversely, a smaller tire means your speedometer reads higher than your real speed. For example, switching from 205/55R16 to 225/45R17 (a nearly identical diameter) causes less than 1 km/h difference at highway speeds.
Within limits, yes. Each rim width has a range of acceptable tire widths. Typically, you can go about 10-20 mm wider or narrower than the recommended tire width for a given rim. For example, a 7-inch wide rim can usually accommodate tires from 195 mm to 225 mm wide. Check your wheel manufacturer's specifications for exact compatible ranges. Using a tire that is too narrow or too wide for the rim compromises handling and can be unsafe.
Fitting tires that are significantly larger than original can cause several issues: the tire may rub against the fender, wheel well, or suspension parts during turns or over bumps; your speedometer and odometer will be inaccurate; ABS and traction control systems may not function correctly; fuel economy can decrease due to increased rolling resistance; and in some regions, oversized tires may cause your vehicle to fail a safety inspection.
"Plus sizing" means increasing the wheel diameter by one or more inches while decreasing the tire's aspect ratio to maintain approximately the same overall diameter. For example, moving from a 205/55R16 to a 225/45R17 is a "plus one" change. This gives you a wider contact patch and improved handling responsiveness while keeping the speedometer accurate and maintaining proper ground clearance. Plus two (e.g., going to an 18-inch wheel) and plus three setups are also common on performance and luxury vehicles.