One on youtube had same issue too, but he found out what was his problem, he drove in Red Bull in GP offline mode so he obviously didnt get that speed without the McLaren and after that he hit 312Kph too, so be sure you dont do that simple mistake
. Another one could be that youre not aware of the engine bug in this game which means that you have to set engine to "fast" manually on the track and not in the setup in the menu. Another one could be that you maybe need to get a better turn before that long straight to be able to hit that speed.
edit: could also be the automatic gearbox or/and traction control, I dont know how much that affect the speed.