And I'm still trying to accomplish it.
You only get the real ending if you finish 1st in all courses with a given car type. Each car type will have its own "ending" (it's just a glorified replay with credits).
As the US version doesn't have a save RAM, you would have to do it in one sit in real hardware. It's pretty doable for the Formula car, easy for the stock car and a real bitch with the prototype one.
Then you still have all the mirrored tracks which are enabled when you finish 1st in all regular tracks with any of the car types. I didn't reach the part of beating the mirrored tracks but I guess that they have their own glorified mirrored replays for each one of the car types.
So, IMO, you need to achieve those six endings in order to consider the game beaten. The gameplay really changes a lot with each car type, so I don't consider beating the game with just one of the types a valid completion, much less if only the regular tracks. The 3rd and 4th tracks are really challenging with the prototype car, but not so much with the others.
If you finish the game only using the stock car, for an example, you'll probably get your ass kicked when using the formula car and even worse with the prototype car.
IDK if that's a overly assine way to see it, but that's how I see it.