Free Tire Size Calculator
Compare tire sizes, check speedometer error, and find the correct plus size.
Current tire (e.g., 225/65R17)
New tire (e.g., 245/60R18)
What is a tire size calculator?
A tire size calculator is a free online tool that helps you compare tire sizes, calculate overall diameter, and see how a size change affects speedometer readings. It runs entirely in your browser — no signup, no app, and no data is ever sent to a server.
The thing that surprises most people I talk to is how much a tire size change affects the speedometer. Going from a 225/65R17 to a 245/60R18 — a very common plus-one upgrade — changes the diameter by about 0.4% and barely matters. But going to 265/70R17 for off-road use can throw your speedometer off by 5% or more, meaning at an indicated 60 mph you're actually doing 63. A tire size calculator compares two tire sizes and shows the exact difference in diameter, plus the resulting speedometer error.
How it works
Diameter = (2 × width × aspect ratio) / 25.4 + wheel diameter. Speedometer error = (new diameter / old diameter − 1) × 100%. A positive error means you're going faster than the speedometer shows. The calculator shows current diameter, new diameter, difference in inches and percentage, and the corrected speed at 60 mph.
When to use a tire size calculator
When upgrading to larger wheels (plus sizing). When fitting bigger tires for off-road or winter driving. When replacing tires with a different size than stock. When buying a used vehicle with aftermarket wheels to verify the speedometer accuracy. When comparing tire options to find the size that keeps your speedometer reading legal and accurate.
Common mistakes when changing tire sizes
The most common mistake I see is plus-sizing without checking clearance. Just because a tire fits on the rim doesn't mean it fits in the wheel well — especially on turns or over bumps. A 265/70R17 might look great on paper and only show a 3% diameter increase, but it could rub the fender lining at full lock. Always measure actual clearance before committing. The second mistake is ignoring load rating. Going from a stock 225/65R17 to a wider 245-series tire often changes the load index, and if you're carrying heavy loads or towing, an underspec tire can fail dangerously. The third mistake I see constantly is mixing tire sizes on the same axle. If you replace only two tires and choose a different size, the diameter mismatch can confuse ABS sensors and traction control systems — the car's computer detects different wheel speeds and may trigger warning lights or reduce power.
I always recommend checking your vehicle owner manual for the OEM tire size before buying replacements. Many cars have different front and rear tire sizes, especially performance vehicles, and assuming they are the same is a costly mistake that the calculator helps you catch before you order.
How to use this calculator
- Enter your current tire size — Input the metric tire size (width, aspect ratio, wheel diameter).
- Enter the new tire size — Input the replacement tire size you're considering.
- Read the comparison — See diameter difference, speedometer error, and plus-sizing recommendations.
Frequently asked questions
How do I calculate tire size diameter?
For metric tires (P225/65R17): section width = 225 mm, aspect ratio = 65%, wheel diameter = 17 inches. Sidewall = 225 × 0.65 = 146.25 mm. Total diameter = (146.25 × 2 ÷ 25.4) + 17 = 28.5 inches. The calculator shows all dimensions automatically.
What is tire plus sizing?
Plus sizing increases wheel diameter while keeping the overall tire diameter the same. Plus 0: larger wheel with shorter sidewall, same overall diameter. Plus 1: 1-inch larger wheel. Plus 2: 2-inch larger wheel. The calculator recommends plus sizes that keep the speedometer reading accurate (within 3% of original diameter).
How much will my speedometer be off with larger tires?
Speedometer error = (new diameter / old diameter − 1) × 100%. If you go from 28.5 in to 30 in tires: (30/28.5 − 1) × 100 = 5.3% error. When your speedometer says 60 mph, you're actually going 63.2 mph. The calculator shows both percentage and per-speed corrections.
What is the metric tire size format?
Example: P225/65R17. P = passenger tire. 225 = section width in mm. 65 = aspect ratio (sidewall is 65% of width). R = radial construction. 17 = wheel diameter in inches. Some tires add a load index and speed rating: 225/65R17 102H (102 = load index, H = speed rating up to 130 mph).
Does tire size affect fuel economy?
Yes. Larger and wider tires increase rolling resistance and aerodynamic drag, reducing fuel economy by 1-3% typically. Heavier tires also increase unsprung weight, further reducing efficiency. Every 10 mm wider can reduce fuel economy by about 1-2%. However, taller tires can improve highway MPG by reducing engine RPM at cruising speed.
What happens if I install tires with a different diameter?
Changing tire diameter by more than 3% from the original causes speedometer inaccuracy, reduced acceleration (larger tires), altered transmission shift points, and potential ABS or traction control issues in modern vehicles. Always check clearance in the wheel well — larger tires may rub on turns or over bumps. The calculator shows the exact percentage difference for any tire change.