I think there is something missing in the question. We can have so many palindromes after 15,951 like, 16061; 16161; 16261 ..... etc.
Joe was riding his motorcycle along a freeway when he noticed that his
bike had 15,951 miles – a palindrome. A palindrome is a number
or a word that reads exactly the same, either forward or backward. He
was surprised to notice that his odometer showed another palindrome
exactly two hours later. How fast was Joe traveling?
Which would yield:
Joe was riding his bike at 55 miles per hour. Is that consistent with the answer if you have one, OP?