Possible Clues:

• Having great energy
• Nothing on the wheel, by the sound of it, that's never worn out
• What 13 across probably is is never worn out
• Your wheel might be, by the sound of it, yet never get worn out
• Never grows weary, inexhaustible
• If your car is like this by the sound of it, you'll have to walk
• Never stopping to 38-Across
• Full of energy
• Inexhaustible novel takes in country and leaves
• Hugely energetic