Estimate the distance change after a tire size change

One revolution, a different nominal distance

An odometer infers distance from wheel rotation and its calibration. If calibration matched the original nominal circumference and stays unchanged, multiplying a recorded distance by the new-to-original circumference ratio gives the theoretical distance traveled. The result uses the same distance unit as the input.

TireSpan calculates circular circumference from the size marking. Existing odometer error, loaded rolling circumference, pressure, wear and recalibration are excluded. This estimate does not establish the accuracy or legal compliance of an instrument.

q = new nominal circumference / original nominal circumference
Theoretical distance = recorded distance × q
Theoretical reading at a fixed distance = distance / q

A 100-mile example in both directions

For 215/55R17 to 225/45R18, nominal diameter changes from 668.3 to 659.7 mm. The ratio is approximately 0.98713. A trip reading of 100 miles corresponds to a theoretical 98.71 miles traveled under the assumptions above. A 100 km reading gives the same numerical result in kilometers.

For a fixed 100-mile road distance, the theoretical reading is instead 100 ÷ 0.98713 = 101.30 miles. The forward distance change is −1.29%; the reading change at fixed road distance is +1.30%. These percentages use different denominators, so reversing the sign alone is incorrect.

Use the distance field in a comparison

Open Compare tire sizes, enter the original and new markings, and calculate. In the odometer section, enter a non-negative trip reading. The distance unit follows the speed table: km/h selects kilometers and mph selects miles. Changing units converts the entered distance. Swapping tire sizes recalculates the ratio.

The field is a temporary trip-distance scenario. Saved projects and CSV, SVG, PDF and JSON exports retain their existing tire comparison format; the scenario is not saved or included in those reports. It does not edit a vehicle odometer or correct a lifetime reading.

When manufacturer data matters

A tire product specification may publish measured revolutions per mile or rolling circumference. Those values include a different measurement basis from an unloaded circular estimate. Preserve the manufacturer, product, measuring conditions and units when comparing them.

Use documented vehicle and tire requirements before changing size. A close circumference ratio alone cannot confirm load capacity, wheel compatibility, clearance or approval.

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