If you put every point you earn into your E class car, you'll get through E class faster.
But then you'll get to D class with no points. You'll be racing a stock D class car, while everyone else (who saved their points from E class) will have some upgrades. You'll have a lot of trouble getting to the point where you can earn points in D class, and you'll be forced to spend your points pretty much right away.
And then, when you do start earning points, you'll probably do the same thing again, and spend all your points on your D class car as fast as you get them. And then when you move to C class, you're back to the same problem, with no points in hand to work with.
The basic rule you want to follow is to spend no points in E class, save them. When you move up a class, spend the points you earned in the previous class. So when you enter D class, use the points from E to do upgrades. Once you've done that, don't upgrade the D class car anymore, it should be plenty to get you to the next class. When you move to C class, spend the points you earned in D class. When you move to B class, spend the points you earned in C. (This is a good place to save some extras. You can get through B without spending everything you earned in C, and it helps to save some because in A class, you really want to have enough points when you first enter to fully outfit a car.)
It sounds to me like you haven't read
my racing guide, and I think it would help you.