New vs Worn Tire Circumference Calculator

This calculator works by computing the circumference of a new tire and of the same tire after a set amount of tread loss. It reports the change in inches and percent.

1. Enter the size, for example 235/65R17 (91.19 in new).
2. Enter the tread lost, for example 8/32 in (6.4 mm).
3. Read the worn circumference: 89.62 in.
4. Read the change: −1.57 in, or −1.72%.

The formula is Worn Circumference = π × (New Diameter − 2 × Tread Lost).

The absolute change is the same for every size at the same tread loss. Worked example: 8/32 in of loss always removes 2π × 0.25 = 1.57 in (39.9 mm) of circumference.

The conversion table compares new and worn circumference for 6 sizes after 8/32 in (6.4 mm) of tread loss. The change is −1.57 in in every row, but the percentage falls from −2.01% to −1.46% as size grows.

Small car tires feel tread loss most in percent. Large truck tires feel it least.

The New vs Worn Tire Circumference Calculator is a tool that compares the rolling distance of a tire when new and after wear. Circumference ties directly to revs per mile and to speedometer and odometer accuracy.

The tool explains why a new set of tires changes the speedometer reading by 1–2% compared with the worn set it replaced.

This calculator has 4 uses: AWD matching, calibration timing, speedometer checks and fleet records.

• Match AWD tires: Measure the gap between new and worn.
• Time calibration: Calibrate on half-worn tires for average accuracy.
• Check speedometers: Explain readings after new tires.
• Keep fleet records: Track circumference over tire life.