They appear to have the same meaning, but full sentences would lead to a better answer.
Interested in Language
I was wondering if these chunks, followed by a verb in the present or one in the past, make any difference in the beginning of a sentence.
What if I told you that...
What if I tell you that...
***** NOT A TEACHER *****
I do not have the answer, but I do have some information to share with you.
1. The New Oxford American Dictionary (2001) tells us that the phrase "what if" means "What would result if ____?" Its example: "What if nobody shows up?" [I emphasized the present tense.]
2. I checked the Web as carefully as possible, and I found one interesting comment in Stack Exchange:
"With a first-person subject, there is not too much difference in meaning, but if I tell you is saying it is quite possible that I will tell you, whereas if I told you is considering a hypothesis, probably less likely."
3. Look at this example from the online Macmillan Dictionary:
"What if the boss walked in here now and saw us?" [My emphasis again]
Compare this with: "What if the boss walks in here now and sees us?"
In my OPINION, the first sentence implies a less likelihood that the boss will actually walk in.
4. Therefore, as the Stack Exchange answer implies, perhaps there is not too much of a difference between (let's say) "What if I tell you that I love you?" and "What if I told you that I love you?"
But I do detect a slight difference, which I am not able to articulate (explain).
Hopefully another member can. I am eager to know.
It does not make much sense to pursue nuance that average speakers would not understand or recognize.
A: You say you like dogs. What if I told you my dog was a pit bull.
B: You say you like dogs. What if I told you my dog is a pit bull.
C: You say you like dogs. What if I tell you my dog is a pit bull?
I don't see much difference here.