With “No estoy segura pero tal vez __ un coche nuevo” both compramos and compremos are considered correct.

Hola K!

The lesson you are talking about refers to the usage of El Subjuntivo and El Indicativo with "tal vez" and "quizás". As explained in the lesson, these two expressions can be followed by both moods and the intensity of probability or doubt is the same, so they are interchangeable without a change of certainty. There are few cases where we can accept both moods without a change of certainty so this lesson is really an exceptional case. However, as you already may know, other expressions only accept the subjunctive or indicative and the nuances are "evident". To be honest with you, there is no 100% accurate rule about how to use these moods all the time, obviously we tend to use the subjunctive when we want to express more uncertainty or doubt. We are planning to do some study lists with expressions followed by indicative and/or subjunctive in order to help students to put some more light on this. Thank you very much for your contribution! Silvia.