Did you change the oil when you first got the car? Because it should have 15xxhp*
My x2010 easily hits 300+ under it's own speed..
Increase your final gear to 4.000, then increase your top speed to MAXIMUM... and go back and decrease your Final Speed.. you can get up to 400mph I think, but obviously the engine can't push that hard. Just make it... 320ish..
My X2010 is set to TOP SPEED 311mph and final gear of 2.815.. giving it an actual speed of 321. It runs 300+ down sarthe(no chicanes)