I tuned my DC5 yesterday for 3K. It was only a 2 minute tune as Iv'e already tuned for 1/4m and 1km but it won every race. I have no idea how fast the other guys were though as I rarely run 3km.
I flipped it at around 5000 (final gear) if I remember correctly with 180kph max speed. Then I set top speed around 330kph. (6th gear)
I think if this still doesn't work I'd look at your suspension settings. Ride hight, springs, dampers, toe & camber.
