I'm beyond level 20 of the first Pimsleur program and I have a couple questions but it's a different release than the one in the topic.

What is the difference between мне and меня? I assumed they were the same word being enunciated differently at first.

скажите мне – say to me.
спросите меня – ask me.

How do I know whether to say

У нас в машине достаточно бензина?
У вас в машине есть бензин?

Does достаточно result in it changing forms?

If you are saying you want to go, do you put в in front of Санкт-Петербург? It doesn't sound like he does in the recording.

The word for car is машина, and I see that cars is машины, then do you pronounce the ы at the end? In the program I do not hear it. I guessed it was машин legitimately.