Hmmm. Since "My uncle's car is faster than my car" means the same as "My uncle's car is faster than mine", I would think Jonathande61's version should be acceptable. Duolingo often accepts varying translations, as long as the meaning is identical.
Duolingo does accept varying translations as long as the meaning is identical. though it only does that as long as all the words in the sentence have been translated. In this case " ..sneller dan de mijne" does mean the same, though the second car is not translated.