How a different tire size changes your speedometer

The nominal speed estimate

estimated_actual_speed = indicated_speed × (new_diameter / original_diameter)

Use the same units for both diameters. This model assumes the original speedometer was accurate and has not been recalibrated. Tire load, inflation, wear, actual rolling circumference, and existing calibration error are outside the calculation.

Worked example: 215/55R17 to 225/45R17

Size	Nominal diameter
215/55R17	668.3 mm
225/45R17	634.3 mm

diameter_ratio = 634.3 / 668.3 ≈ 0.94912
at an indicated 60 mph:
estimated_actual_speed = 60 × 0.94912 ≈ 56.95 mph

The replacement is about 5.09% smaller in nominal diameter. Under the stated assumptions, the estimated actual speed is about 56.95 mph when the display reads 60 mph. This is a geometry example, not a replacement recommendation.

Distance uses the same ratio

For a fixed indicated distance, the estimated actual distance changes by the diameter ratio. To express the odometer's reading error relative to actual distance, use the inverse ratio:

actual_distance = indicated_distance × diameter_ratio
odometer_reading_error_% = (1 / diameter_ratio - 1) × 100

In this example, an indicated 100 miles represents about 94.91 actual miles. The odometer over-reading relative to actual distance is about 5.36%. It is not exactly the negative of the diameter percentage because the denominator differs.

How to read the direction

Replacement diameter	Estimated effect, with original calibration
Larger	More distance per revolution; actual speed and distance are higher than indicated.
Smaller	Less distance per revolution; actual speed and distance are lower than indicated.

What the estimate does not establish

The ratio does not establish legal speedometer compliance, vehicle-system compatibility, or physical fitment. There is no universal diameter percentage that verifies those details. Use manufacturer guidance and a qualified installer for a size change.

Compare your two sizes to see the nominal diameter change, estimated speed effect, and dimensional drawing together.