And so Gaby, you answer the question yourself.
I believe it's possible to gag you so that the sounds from your mouths gets silenced quite a bit - still sounds will leave your body. If the gag should take away most sound, I believe you should be strapped up very strict - and this might be risky in terms of hurting your jaw or even worse cause you to suffocate. I'm pretty sure your husband/Master would indeed prefer seeing a living submissive/slave when openting the car booth. So real life does indeed set some restrictions on how strict the gagging can be.
Then lots of modern people are biased towards expressing things verbally. This is especially true with women, who from the early puberty has outsmarted the boys verbally. Guess that's why men tends to beat the women - they would never win an argument
So if you can't express sensible and especially understandable words, you will enjoy a severe handicap - maybe even more than when being in bondage.
By having taken two action-items from your arsenal, you'll feel more vulnerable - which I believe is one strong plus for you.