Friends who can sit in a quiet car without talking usually learned to sit in quiet by themselves first.

Comfortable silence between people is not an absence of conversation, it is a sign that neither of them needs the other to fill anything. That usually comes from having sat in quiet alone often enough to stop hearing it as a problem. People who find their own company unbearable tend to keep talking.

Try leaving a natural silence alone next time it appears, instead of reaching for a question. See how long it takes to feel fine. Friendships that survive that test are worth more of your hours than the ones that need constant noise to stay upright.