I'd like to know when I can say "说差（了）". My dictionary says the "差" means 错误 which means "wrong" in English but I'm still not clear on the usage of 说差. Is it the same as 说错? It would be appreciated if you give me some examples like short conversations that show how 说差 is used. Thanks in advance.
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!
说差了 could mean what you are saying is somewhat right but not totally right. It has the similar meaning to 说对了一半, which means you are only half right.
“Tom今年十八……呃，不对，十七岁。” (Tom is 18... hmmm, nope, 17 years old.)
找您五毛……啊，错了！四毛。 (This is your change --50... Uh! Nope, 40 cents.)
Native Chinese speakers don't use this phrase in daily life, If you want to tell somebody "what you said is wrong.", you can use “说错了” or “错了” but not "说差了". Though there is meaning called “wrong” in bunches of meanings of "差", which usually represent "not good, less good than something else.", we don't use it frequently.
By the way, i have never used the phrase "说差了". I'm a Chinese student, native Chinese speaker.
In most circumstances, we don't say "说差了".
BUT in Shandong accent, we often use "差(cha4)" instead of "错". ---- So that @Atmega 328's answer ——
Chinese people never use "说差" at all.
—— is proved wrong.
At last I strongly suggest you use "错" rather than "差" because it's commonly used anywhere in China.
There are black and white, and also grey. 差 is the grey area where you are not totally wrong nor totally right. For example, if a Chinese call you a 老外 (old foreigner)，then，you can say：你说差了，我不老，我只是外国人。 or you can say it like this, same meaning, but more sophisticated : 君此言差矣，寡人并不老，人家只是一名外国人！
also Chinese people never use "说差" at all。 It's not a thing in Chinese. We use: 此言差矣 in situation where someone has said something slightly wrong.