Tire wear reduces effective rolling diameter, overestimating speed

Steps:

• Original tire diameter is 600 mm, so circumference is π × 600 mm.
• 6 mm wear on outer surface reduces radius by 6 mm, decreasing diameter by 12 mm to 588 mm.
• New circumference is π × 588 mm.
• Speedometer uses original circumference; for same rotation rate, shown speed = actual speed × (600/588) ≈ 1.02 (2% higher).

Why B is correct:

• Speed is proportional to wheel rotation rate times circumference; smaller actual circumference means shown speed exceeds true speed by (original diameter - new diameter)/new diameter = 12/588 ≈ 2%.

Why the others are wrong:

• A: Error is 2%, not 1%.
• C: Speed shown is too high, not low.
• D: Speed shown is too high, not low, and error is 2%, not 1%.

Final answer: B