Yes, this is grammatically correct, although it sounds a little over the top to me. If you at the 'buddy' level, the intro 'I don't know how to say this without sounding rude' sounds strange. 'I'm sorry, sir, I don't know how to say this without sounding rude, but your car isn't quite as good as you want to believe it'. Otherwise, I would just say: 'Sorry, buddy, but your car looks like a lemon to me'. I hope this helps Jo.

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