'Your car will be ready by then' is clearer in this situation. Your car will be fixed by then is similar because in this case 'fixed' is an adjective, like 'ready' (it was 'broken' before) If you talk about the action, 'fix', you can say 'will have been fixed' - this is a verb in the passive form. You can say that but the other way is more natural in this case. :)

They are both saying the same thing. Your car will be fixed tonight. Will be fixed = some people prefer this short way of speaking. Will have been = some people prefer this way of speaking. They are both a promise guarantee commitment that when you go back that evening your car will or will have been fixed.

You probably want to use will be. This refers to to a point in time which is in the future relative to now - in this case the year 2030. Will have been refers to a time, which is in the past relative to a time in the future.