Financial issues definitely put stress on a marriage. Sometimes my husband starts thinking about a big purchase that doesn't make any sense and if I say anything he will blow up about it at first....but then starts thinking about it. The hardest thing to do is to communicate without hostility and resentment. Sometimes husbands will think that they don't need to discuss big decisions with wives because they are the 'head of the family'. Yet, wives can, many times, give good solid input and they really need to be willing to at least listen to it. Especially since these decisions affect you and your children's lives too.
Wives need to be very careful when discussing their in-laws...especially if the husband is immensely 'loyal' to his parents. That can be good or bad depending on the situation. If I were you, I'd be careful about any criticism towards your in-laws. Is their situation critical? Do they really NEED a new vehicle...and is he talking about a brand new car? There are lots of dependable used cars on the market.
One thing that might help him listen to you is if you're willing to compromise ...like sitting down, going over your finances and deciding on an amount of money that you can contribute to their car purchase. He may be reacting to your hostility toward his parents instead of what you're actually saying. I know from my own marriage to NEVER criticize my mother-in-law in any way, shape, or form. I'm glad my hubby honors his parents and I want to support him in this as much as possible. At the same time, your own family has to be able to make ends meet.
Praying for you both...patience, calmness, and the leading of the Holy Spirit in your relationship. Before speaking to him, have a prayer time of confession, repentance, and seeking God's guidance.