roberto, i'd like to answer a couple of your questions.
firstly, you can recite your mantras when you drive but it cannot be counted as part of your retreat. Not only because your retreat seat should not be moved throughout the duration of your retreat, but also because when you are driving, your attention is not entirely focused on your prayers.
Proper retreats involve a fair bit of visualisation, meditation, focus, concentration etc so when you are driving, either you are only partially focused on your prayers, or you are very close to having a major car accident.
My lama has also told me once, that when you recite mantras while doing another activity, the mantra count is actually halved. So if you recite 10 malas on your drive home, it only counts as 5 malas cos you're not focused on just the mantras.
2nd thing - you have got to recite audibly, not just think it. It has to be loud enough for yourself to hear at the very least. In fact, the best way to recite your mantras is loud enough for you to hear, but soft enough so that no one else can hear. That also encourages humility, as you are not showing off your practice.
hope that helps