Struggling
I have a nearly 15 year old step daughter. She lives in the United States while my husband, my bio daughter, and our son live overseas with the military. My husband's relationship with his bio daughter has changed since we were married but I think it is the typical evolution of things. She was becoming a teenager and thus more distant from adults anyway, we moved from a 12hour drive to her home to a 36hour drive from her home (and a 4hour drive to the closest airport), and my husband has deployed multiple times since we married. I don't think our marriage in and of itself has been the reason he has been unable to visit her as often as he did before we married. Her mother refuses to let her fly alone, so in order for my husband to see her he has to go to her home, which he always does alone because we cannot afford to fly all of us back stateside on a regular basis. If she is to come overseas her mother says that my husband has to come get her and then fly her back to our home and then bring her back stateside again, which adds up to 3 round trip international tickets. Needless to say, she hasn't visited us in nearly 3years 
I have always tried to stay out of the relationship between my husband and step daughter. It was difficult at first, to feel so completely excluded but I have been able to learn to respect that while I consider her and I friends, she really just wants time with her dad when possible. Recently, I learned that my husband's ex wife got in touch with him about a sweet sixteen party for their daughter (bio mom is MUCH more affluent than we are). She hasn't asked for money for a party that is being planned 15months in advance but I think its coming. Then bio mom asked my husband about giving our car to their daughter for her 16th birthday. Our car is currently being stored by the military in the US until we return in two years. The military only allows us to store one car while we are gone, so we sold our other car with the intention of buying one new car when we returned, paying it off and then most likely replacing the stored car once we had paid off the new car (geez, i hope this isn't getting to confusing). Well today my husband talked to his bio daughter (he is currently deployed) and she asked if she could have his car when she turns 16. He said yes, that he would find a way to get it out of storage and she could have it. He hasn't talked to me about this at all, I only know because of facebook. good ol' facebook. He rarely consults me about issues regarding his daughter, even when it effects our family. Now we will be coming back to the US with no car and a need for two. He will have to get to work everyday and I will have two kids to shuttle around. I am at a loss and I don't know what to do. We can't afford to buy two cars at once without dipping into college funds and I am scared. How do I deal with this with out sounding like the horrible step mom? He is very defensive when it comes to his daughter and my opinions on things, one more reason I keep my mouth shut about it. You know how Social Security is the third rail of politics? Well my step daughter is the third rail of my marriage. Touch it and you'll be electrocuted.
I would try to talk to your
I would try to talk to your DH about it, but be careful about your approach. You are correct that his agreeing to give your only car away will certainly leave the two of you in a bind once you return stateside. Is he going to have time to go purchase two cars before he has to report? I understand he's thinking of his BD, and probably thinking that the car is just sitting there in storage but he's not thinking of the long-term of when you will return to the states.
My DH sometimes "forgets" (snort) to talk to me about things like this too. I pick my battles, but if it is something that will financially or emotionally impact me or my family, you bet your bottom dollar I'll be asking about it! If he wants to make a decision that will not alter me, my kids, or our way of life I don't really care, but anything else (big $$ expenditures, massively flipping schedules, etc) BETTER be discussed with me. It's what married people (are supposed) to do and it's just the right thing to do.
And I probably would mention that it was a little bit disconcerting to read about these plans on facebook (the downfall of our society as we know it) and that social media knew about it before you did. That's the sarcastic person in me coming out...
Maybe it is different in the
Maybe it is different in the US, but here in UK, very few people would think it OK to give a 17 year old a car for their birthday (kids cannot drive until 17 here). Quite apart from the fact that insurance costs are prohibitive - most people do not have that kind of money. If your SD's BM is wealthy and she thinks a car is an appropriate present for a 16 year old, let her give her one.
Your husband firstly may not want to be outdone gift-wise by his rich ex, and secondly feels guilty that he doesn't see his daughter much, so feels he must agree to this request. However, he doesn't have to, and there is no excuse for not talking to you about it first. My DH used to do things unilaterally without running them by me first, but although I am cool about most things he does, I made sure he understood that this was an absolute no-no and I was not prepared to put up with it.
If your husband is anything
If your husband is anything like my BF, then he's likely to get defensive about things when you talk to him about his daughter (he'll take it as you criticizing him) and he probably does things like that (giving her the car) as a way to keep the peace with her.
As for him having to travel to pick her up...my boyfriend's two kids have been flying solo for years (since they were 4 and 6). All you have to do is say she is an unaccompanied minor, which will be the case until she turns 16 and depending on the airline. If she's old enough to want a car then she's old enough to fly alone. Paying for an unneccesary plane ticket, I suspect, is the ex-wife's way of being a pain in the ass.