When can I say "说差了"?
Usually we just say 说错了 if we found we made a mistake in a talk or statement. 说差了 is not a common usage for 'I made a mistake' regarding to what you said. I wonder if you actually want to discuss about 说岔了 instead of 说差了. 说岔了 usually means 'misunderstanding on either the topic or the opinion caused by confusing words/expression/context'. For example, when a group of friends are discussing about 吃醋 in a context to compare which vinegar is the best; you jumped in and told them that your girl friend likes 吃醋 (be envy of other girls you know). They may told you that 你说岔了!