Tire Size Calculator & Guide
Instantly compare tire dimensions, calculate speedometer error, and ensure safe wheel and tire fitment for your vehicle.
Compare Your Tire Sizes
Enter your original factory tire size and your proposed new tire size below to instantly see the dimensional differences and speedometer impact.
Tire Size Comparator
Check dimensions and speedometer accuracy.
Understanding Tire Sizes
Decoding the sidewall markings
Tire Width (mm)
The first number (e.g., 205) is the width of the tire’s tread from sidewall to sidewall, measured in millimetres.
Aspect Ratio (%)
The second number (e.g., 55) is the sidewall height expressed as a percentage of the tire’s width. A 55 aspect ratio means the sidewall is 55% of 205mm.
Rim Diameter (in)
The final number (e.g., 16) is the diameter of the wheel rim that the tire is designed to fit, measured in inches.
The 3% Rule
When changing sizes, the new overall diameter should not differ from the original by more than 3% to maintain speedometer accuracy and vehicle safety systems.
Common Plus Sizing Equivalents
Popular alternative tire sizes that maintain an overall diameter within the safe 3% tolerance range.
| Original Size | Alternative Size | Diameter Diff | Speedo Error @ 60mph | Fitment Note | |
|---|---|---|---|---|---|
| 205/55R16 | 215/50R17 | +0.4% | 60.2 mph | Safe “Plus One” upgrade | |
| 225/45R17 | 235/40R18 | -0.3% | 59.8 mph | Safe “Plus One” upgrade | |
| 215/60R16 | 225/55R17 | +0.2% | 60.1 mph | Safe “Plus One” upgrade | |
| 235/55R18 | 245/50R19 | -0.5% | 59.7 mph | Safe “Plus One” upgrade | |
| 205/55R16 | 225/50R17 | +1.6% | 61.0 mph | Wider, slightly taller. Check clearance. | |
| 215/65R16 | 225/65R16 | +1.4% | 60.8 mph | Same rim, wider tire. Slightly taller. |
Tire Sizing FAQ
Answers to the most frequently asked questions about tire dimensions, fitment, and vehicle performance.
A tire size like 205/55R16 means: 205 is the tread width in millimetres, 55 is the aspect ratio (sidewall height as a percentage of width), R means Radial construction, and 16 is the rim diameter in inches.
When changing tire sizes, the overall diameter of the new tire should not differ from the original factory tire by more than 3%. Exceeding this can cause inaccurate speedometer readings, affect ABS and traction control systems, and cause the tire to rub against the wheel well or suspension components.
Plus sizing involves increasing the wheel (rim) diameter while decreasing the tire’s aspect ratio to keep the overall tire diameter roughly the same. For example, moving from a 16-inch to a 17-inch wheel, but using a lower profile tire (e.g., from 55 series to 50 series) to maintain the correct overall circumference.
Yes, generally larger or wider tires increase rolling resistance and unsprung vehicle weight, which can slightly decrease fuel economy. They also alter the effective final drive ratio, which can impact acceleration and increase engine RPM at highway speeds if the overall diameter is significantly larger.
It is highly recommended to use the exact same tire size, brand, model, and tread pattern on all four wheels. Mixing sizes, especially on the same axle, can cause severe handling issues, uneven wear, and damage to all-wheel-drive (AWD) or four-wheel-drive (4WD) drivetrains.