How can a motorcycle rider determine the manufacturing date of a tire, and why should tires older than 5–6 years be replaced regardless of remaining tread depth?

1. By checking the 4-digit DOT date code stamped on the sidewall (e.g., '2422' means week 24 of 2022); aged rubber compounds oxidize, harden, and lose chemical grip over time
2. By measuring the tire circumference with a tape measure; older tires shrink by 10% each year
3. By scratching the tread with a key; older tires change color from black to fluorescent yellow
4. By smelling the valve stem; older tires release sulfur aromas when inflated