I agree with the other posters (except in every jurisdiction I’m aware of, cops will love you to start talking in the car – they’ll just Mirandize you as soon as it starts).
The Miranda warnings have no effect on the legality of an arrest or prosecution generally; they only pertain to certain types of evidence – namely, confessions or evidence obtained as the result of something the suspect told them. They are meant to protect the suspect who is in custody from thinking he is required to answer police questions. They are an outgrowth of the 5th Amendment, which prevents the state from forcing a suspect to testify against himself. The theory is that even if the cops don’t say so directly, the trappings of custodial interrogation tend to make suspects think they have to talk. So, as a prophylactic measure, the cops essentially have to tell anybody in custody that they want to self-implicate that he doesn’t have to do that – that he can just shut up if he wants. But if the cops feel the case will make without a confession, they don’t have to give you the warnings because they aren’t looking to use anything you tell them.
When you hear of someone getting off because he wasn’t Mirandized (whether in TV or real life), the upshot is that the case was based on a confession (or evidence gathered which the cops knew to look for because the suspect told them about it) that was illegally obtained without a warning.
That said, even in live situations of custodial interrogation, the Miranda warnings are likely given much less in real life than on TV. On TV, it’s a cliche. In real life, an allegedly common procedure is for the cops to ask someone to come in, then put him in a room and intimidate him for several hours, asking him questions. If he talks, the cops say it wasn’t a custodial interrogation b/c he was free to leave at any time and, as such, the Miranda warnings weren’t constitutionally required. If he decides to leave, the cops tell him he’s under arrest, and then they do give the Miranda warnings and continue the interrogation. Without knowing more about the matter, I’d say it’s possible the cops may have been running this sort of scheme when you were arrested.
N.B., the Miranda warnings aren’t absolutely required even if the cops want to use confession evidence. The Supreme Court created them out of whole cloth because it felt that something needed to be done to protect defendants’ right against self-incrimination when the cops had so much more power, stagecraft, and ability to learn exactly what the law does and does not allow/require. But if a jurisdiction wanted to do something even more protective of defendants, they could. For instance, if the jurisdiction just gave everyone a lawyer immediately upon arrest instead of merely telling the suspect that they could get one, that would suffice, and no warnings would be required.
–Cliffy