Now this is a problem indeed. Let's try.
много машин - there are some cars and they are many. That's it. Just many (or "a lot of").
многие машины - this phrase always implies that there are other cars beside mentioned. It is "a big part of the whole" rather than just "many". Often it is used like "многие машины из тех, что были угнаны позавчера" that is "many of".
So.
По улице ехало много машин. - It is about the street, that contains some cars.
Многие машины ехали по улице. - It is about cars you were talking before. Many of them were moving along the street.
Though sometimes this difference is slight and expressions can be interchanged... but sometimes no.