Tire Size Speedometer Error Calculator

Calculate the speedometer discrepancy and safety margin when changing tire sizes. Compare stock vs. new tire dimensions visually.

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Category Car Calculators

Stock Tire (Factory)

/R

New Tire (Aftermarket)

/R
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About

Changing your vehicle's tire size is not just an aesthetic choice; it fundamentally alters the geometry of your drivetrain. When you install larger or smaller tires than the factory stock, the overall circumference of the wheel changes. Since your speedometer measures speed based on the rotation of the axles—calibrated to the factory tire size—a change in diameter results in incorrect speed readings.

This inaccuracy can have real-world consequences. A speedometer reading lower than your actual speed puts you at risk of inadvertent speeding tickets. Conversely, significant differences can confuse your vehicle's computer, potentially affecting Anti-lock Braking Systems (ABS), traction control, and transmission shift points. Generally, a difference of less than 3% is considered acceptable, while anything above that requires recalibration or caution.

tire calculator speedometer error tire comparison car safety tire diameter

Formulas

The calculation of the tire diameter involves the section width, aspect ratio, and rim diameter. The formula determines the total height of the tire assembly.

D = (W × AR100 × 2) + (R × 25.4)

Where D is Diameter (mm), W is Width (mm), AR is Aspect Ratio, and R is Rim size (inches). The factor 25.4 converts the rim from inches to millimeters.

The speedometer error percentage is calculated as:

%Error = (Dnew − DoldDold) × 100

Reference Data

Tire SizeDiameter (mm)Diameter (in)Circumference (mm)Revs/km
205/55R16631.924.881985.2503
225/45R17634.324.971992.7501
245/40R18653.225.722052.1487
255/35R19661.126.032076.9481
275/40R20728.028.662287.1437
195/65R15634.524.981993.3501
315/70R17872.834.362742.0364
265/75R16803.931.652525.5396
235/50R18692.227.252174.6459
215/60R16664.426.162087.3479

Frequently Asked Questions

Your car's speedometer calculates speed by counting how many times the wheels rotate per minute. It assumes the factory tire size. If you install larger tires, they travel a longer distance per rotation, meaning your actual speed is faster than what the gauge shows.
Most automotive experts recommend staying within a 3% difference of the original tire diameter. Exceeding this limit can cause brake failure warnings, poor fuel economy, and transmission issues because the vehicle's computer can no longer accurately predict the load and speed.
Yes. Larger tires are generally heavier and have more rolling resistance, which requires more energy to move. Additionally, because the effective gear ratio becomes 'taller', the engine may operate outside its most efficient RPM range during city driving.
If the difference is significant (usually over 3-5%), recalibration is highly recommended. This can often be done via a handheld tuner or by a professional mechanic to ensure your odometer and speedometer remain accurate.