The English idiom "as if!" is used to express the speakers rejection that something is possible:
A: "In a few years I'll be driving a Ferrari."
B: "As if you'll ever have a Ferrari!"
Here, B, does not believe it is possible that A will ever own the car.
Someone suggested 废话? What is the best equivalent?
I would suggest 就像真的 is the closest translation. First, it is literally close to "as if". Second, it also has the implication of disbelief.
In your example:
A: 过两年我就开法拉利了。
B: 说得像真的一样 (means 别做梦了)。
"废话!" literally means "redundant utterances". It's the of equivalent somthing like "You bet! [with an unpleasant tone]" which implies you actually agree with him. That's diametrically opposite.
The closest answer I can think of is "才怪".
A: "In a few years I'll be driving a Ferrari."
B: "As if you'll ever have a Ferrari!"
A: 过几年我就能够驾法拉利了。
B: 你能驾法拉利才怪!
怪 means incredible in English 惊异，感到奇怪
才 means having the ability (能力：～能。口～。这人很有～干)
This phrase 才怪 is evolved from the longer phrase "才奇怪" and the usage of which only becomes popular in recent times. If you are interested, you can read up this article which describes at length the historical development of the phrase.
Quoting from the article,
才怪逐渐演变出了一种新的意义：表否定。“否定”的意义是与“奇怪”义有紧密联系的，觉得某事奇怪，主观上本身就有不赞成之意。只有主观上认为与自己看法不相同才会觉得奇怪，出乎意料。所以，我们认为“才怪”现代常用表否定的意义是从“才奇怪”演化而来。
Translated to English:
才怪 slowly evolves to mean an expression of disagreement. The words "disagreement" and "incredible" are closely related. To think that something is incredible implies that the person does not agree that it will happen. Only when a person disagree with something would he find it strange or out of the norm. Therefore, we believe that the contemporary meaning of 才怪 is an expression of disagreement and evolves from the longer phrase 才奇怪.
In my own words, 才怪 has the same exact meaning as "as if it would happen".
Usage example:
A: I'm not going to smoke again, I promise.
我绝不会再抽烟了，我向你保证。
B: As if this is going to be your last cigarette.
这会是你的最后一根烟才怪！
How about "拜托!" ? Which I would use to say 'come on!' Or 'puleeeeze' (an exagerated please)
肯定是好像``正如的意思, 在您的例子中, 那句话最地道的中文说法应该是:说的跟真的似的!
When we trans as if into Chinese, We always use 好像,正如. But in different sentence, we will choose a best fit word. In that example, B think A is 做梦(空想), so, the 说的跟真的似的 表达了B对A的想法的嘲笑的态度.
A: 过两年我就有法拉利了。
B: 你可拉倒吧
任何讽刺吐槽的词汇都适用
How about adding 个屁 to the end of the last thing they said? It can be rude, or very funny :) Sorry for the brief answer, but my written Chinese is almost non present.
A: "In a few years I'll be driving a Ferrari."
B: "As if you'll ever have a Ferrari!"
I suggest 再说吧
A: 过两年我就有法拉利了。
B: 等你将来真有法拉利的时候再说吧！
How about adding 吹牛 to the end of the last thing they said? It can be suitable.
First, it is literally close to "as if". Second, it also has the implication of disbelief.
As an aside, I think the Cantonese equivalent would be 咪呀嗎/唔係呀嗎. The way I've seen/heard it used in HK and China looks like a good match.
A.过两年我就能开法拉利了。
B.好像法拉利是你家造的一样。
好像...一样，好像...似的；between them and “as if” have same means.
