He asked me whether/ if

[Vladv1]

"He asked me whether/ if I liked tea"-are they completely interchangeable in reporting? Any subtle difference?
But here they are not " He asked me me whether to buy this car or not".

 

[emsr2d2]

"He asked me whether/if I liked tea."

Are they "whether" and "if" completely always interchangeable in reporting reported speech? Is there any subtle difference?

Note my corrections above. There is no difference in meaning in that context.

But here In they are not "He asked me me whether to buy this car or not", they are not.

That's because you've followed "whether" with a to infinitive. In the original sentence, that wasn't the case. Also, "He asked me if to buy ..." isn't grammatical.
"He asked me whether/if he should buy this car" would be correct and both words could be used.

 

[Annabel Lee]

"He asked me whether to buy this car or not".

I can't say that I find that sentence fully grammatical. It's supposed to mean, "He asked me whether he should buy this car or not," isn't it? Normally, "whether to . . ." is found directly after a verb, e.g., "He didn't know whether to buy this car or not." The only exception I can think of is with "tell." For example, "Tell him whether to buy this car or not" would, I think, be a grammatically acceptable substitute for "Tell him that he should buy the car or that he should not buy the car."

 

[tedmc]

"He asked me whether to buy this car or not".

Without being specific, the potential buyer of the car is unknown.

 

[Tarheel]

I like iced tea. How would you tell somebody I said that?