Consider the listener's perspective. After hearing your sentence, does he know (a) precisely what amount of time is involved (a quantity, with units), or (b) what event precipitated the beginning of the time period? Even if you say, "I've been spending 3 times more on gasoline since I've had this car," what we know is that the time period began when you got the car, but we have no idea how long that time period was.
I thank you for your comments about my question. I'm delighted people are interested to master english language. It's very interesting to read your comments.
Indeed, the best way to master the grammar is the daily practice as you said. The difference between "for" and "since" is also the same like "like" and "as". That requires the practice in order to master the rule. The simple rule doesn't work.
No, he doesn't know what event started the time period. Perhaps I bought the car, perhaps I inherited it, or acquired it in some other way. The sentence doesn't talk about any event.
The fact that it does not specify the exact length of time is irrelevant. Consider "I've had this car for I don't know how long." -- this is still a sentence that refers to a period of time.
I hope I'm providing constructive dialogue and not coming across as argumentative.
The sentence refers, tacitly, to your acquisition of your car. "This condition began when Tom acquired the car" is the direct reason that `since' should be used. "I don't know how long it's been" is the indirect reason; that means that `for' couldn't possibly be appropriate.
If I may suggest another trick for helping make the decision when your intuition fails you: when you consider your scenario, ask yourself, "When did this situation begin?", and then answer the question. If you can phrase your answer in the form ' .... ago' then the `for' construction is appropriate. Otherwise, `since' is appropriate. Just another rule of thumb; I don't know if it helps. Counterexamples, as always, would be welcome.
This discussion interests me because I wasn't even aware that it's a difficulty for students of English. There are some rules and grammar issues that even native speakers do poorly with. But I've never heard -- and can't imagine hearing -- a native speaker err on the for/since distinction. To be honest, I'd never even considered the two words similar before! So it's intriguing.