you probably have the gears set to high. if the revs arent reaching the power peak in sixth gear then you need to lower the gears. if however you have the gears set all the way (auto set 40-45) and your still hiting redline in sixth gear then I have no clue what the problem is.
No, that wouldn't be the problem, because he said that he maxed out his gear ratios.
It will hinder even more if the gear ratios are maxed out. More downforce = more drag = lower top speed. I am guessing that you are using the test track to get a top speed. Lower the gear ratios to just over 3/4 of the way along and then test it. If you are hitting the rev limiter in 6th gear then they are too short and push them out a bit. Keep doing this until the needle wont go any further whilest deep in the red in 6th gear. Also make sure you have minimal downforce.
