Past simple vs present perfect in this example

Question

Could you explain to me please what is the difference between these. It is meant to express that I will let him know AFTER I picked/have picked a car.

I'll let you know which one I picked

I'll let you know which one I've picked

The present perfect, its because its shifted from future present?

Answer 1

Neither of the above is really fluent. The most natural sentence is actually:

I'll let you know which one I pick.

As you noted, the main clause is in the future tense. Subordinate clauses which are occurring at the same time as a future-tense main clause generally use the simple present.

They'll go inside when it rains.

The birds will migrate when winter comes.

You will need a jacket if it snows.

Answer 2

This question keeps changing!

In its present form, both are acceptable depending on the circumstances.

If your choice of car is relevant now, if somebody is buying it for you for example, "I'll let you know which one I've picked" is the better choice.

If your choice of car is not really relevant, you are just passing the information to keep a friend informed, then "I'll let you know which one I picked" is okay.

Answer 3

They have similar meanings, but are used in different circumstances. Given your clarification that the picking has already taken place, I'll let you know which one I picked means that at some time in the past I picked one and that at some time in the future I’ll let you know which one it was. The second construction would be used in a situation in which whatever I picked was a current topic of conversation, concern or interest.

Answer 4

As you've noted, it's a matter of tense. There are multiple usages, but perhaps the most common refers to the listener's (presumed) prior knowledge.

"I'll let you know which one I picked"

The above sentence would likely relate to a specific act of picking. The speaker has presumed that the listener likely knows that the speaker has picked something, and is telling them what it is. In the case of your example, the listener already knows that the speaker was going to pick a car.

"I'll let you know which one I've picked"

The above sentence would likely relate to a non-specific act of picking. The speaker has not presumed that the listener knows that the speaker has picked something, and is telling them that they have picked something, and will tell them what that something is. In the case of your example, the listener did not already know that the speaker was going to pick a car.