I put the 4500rpm version in set gear box to 200 swapped the 9000rpm one in ran the test and got 275 top speed.
Hey JussiE hope this one meets your requirements 
Anyway I think look at what can be done, numbers, performances, etc, is good to take some kind of references, interesting in some way…
Ah, I thought the game was crashing at some point. I suspect that what has happened is that the gearbox is just too wide at the top end for your car to accelerate in its top gear effectively. try moving the other gearbox slider so that all of the gears are bunched up at the top speed end of the chart as much as the game will let you (and using 9 gears will also help). Another thing you can do is to remove any fixtures giving you downforce because they will massively hinder your top speed. if you need more grip then add some more weight since that doesn’t affect your top speed negatively, just how long it takes to get there. If all of that doesn’t work then my only thought is that the car just wont do 400mph and that you need to lower the gearbox a bit