Here ya go in a nutshell.
Final gear is the key.
Lets say for instance that your max final gear is 5.000 and the min is 2.500. With the final gear set at 5.000 1st gear will be, when set all the way to the left, lining up with the 8 in the red 7800rpm in your transmission graph with a top speed of 168mph. If the final gear is set at 2.500 1st gear will be pointing to the m in the red 7800rpm, when set to the far left. while all the while still at 168mph, still in same location. having the lower final gear makes all your gears closer together which will allow you to say in the best rpm range for your car. A low final gear will have your gears farther apart so you will not stay close to your optimum rpm. Thats why you get "pulled" on at the back half of the race.