Okay, you should use a loop then, which increases the car's velocity by just a little bit each time, so it looks like a smooth acceleration:
#accel
_inc = 1
_car setvelocity [(velocity _car select 0) + _inc*cos getdir _car, (velocity _car select 1) + _inc*sin getdir _car, velocity _car select 2]
? speed _car < 300 && alive _car : goto "accel"
Just change _inc to a higher number and the car will accel faster.