Speedometer Calibration Calculator

Calculate speedometer error after changing tire size. Determine actual speed vs indicated speed, correct odometer readings, and assess calibration needs.

Stock tire width

Stock tire aspect ratio

Stock wheel size

New tire width

New tire aspect ratio

New wheel size

Positive = speedo reads slow, negative = reads fast

What your speedometer shows

Positive = speedo reads slow, negative = reads fast

Your true speed (from GPS)

Speedometer Error Formula: Error % = ((New Diameter - Old Diameter) / Old Diameter) * 100 Actual Speed = Indicated Speed * (1 + Error% / 100) Indicated Speed = Actual Speed / (1 + Error% / 100) Tire Diameter = Wheel Diameter + (2 * Sidewall Height) Sidewall Height = (Tire Width * Aspect Ratio) / 100 Odometer Error = Same as speedometer error
Example 1: Old: 225/65R17 = 28.52" diameter New: 265/70R17 = 30.61" diameter Error = (30.61 - 28.52) / 28.52 * 100 Error = +7.33% At 65 mph indicated: Actual = 65 * 1.0733 = 69.8 mph WARNING High error - calibration needed Example 2: Old: 225/65R17 New: 245/60R17 = 28.57" Error = +0.18% At 65 mph -> 65.1 mph actual Excellent - no calibration needed

Why does changing tire size affect my speedometer?

Speedometers measure wheel rotations, not actual speed. Larger tires = fewer rotations per mile, so speedometer reads slower than actual. Smaller tires = more rotations, speedometer reads faster. Example: 3% larger tires means speedo shows 60 when you're actually going 61.8 mph. This also affects odometer accuracy.

How much speedometer error is acceptable?

Keep error within ±3% for accuracy and to avoid ABS/traction control issues. Most manufacturers allow up to 5% from factory. Legal limits vary by country (US typically allows +10% -0%). Over 3% may trigger check engine lights, affect transmission shifting, and cause inaccurate speed readings that could result in tickets.

Can I recalibrate my speedometer after tire change?

Yes, several methods: 1) Electronic tuner/programmer (best for modern vehicles). 2) Replace speedometer gear (older mechanical speedos). 3) Speedometer calibrator device. 4) ECU reprogramming at dealer. Some vehicles auto-calibrate via GPS. Cost ranges from $50 (gear) to $400+ (tuner).

Will larger tires affect my odometer?

Yes, same as speedometer. Larger tires = odometer under-reports mileage. 3% larger tires means odometer shows 10,000 miles when you've actually driven 10,300 miles. This affects: resale value documentation, warranty claims, service intervals, lease mileage limits. Keep records of tire changes for documentation.

How do I calculate correct speedometer reading?

Actual Speed = Indicated Speed × (New Diameter / Old Diameter). Or use: Actual Speed = Indicated Speed × (1 + Error%). Example: +3% error, speedo shows 65 mph → actual speed = 65 × 1.03 = 67 mph. Use GPS speedometer app to verify real-world accuracy.

What about changing wheel size without changing tire diameter?

If total tire diameter stays the same (plus-sizing: larger wheels + lower profile tires), speedometer remains accurate. Example: 225/65R17 (28.5" diameter) → 245/55R18 (28.6" diameter) = only 0.4% error. Always calculate total diameter, not just wheel or tire size individually.

Do different gear ratios affect speedometer accuracy?

Changing differential or transmission gears DOES affect speedometer on older mechanical systems. Modern electronic speedometers use wheel speed sensors and are not affected by gear changes. However, incorrect tire size combined with gear changes compounds error. Always recalibrate after major drivetrain modifications.