why does it top out at 170 km per hour?

The reason behind that is probably because the car has a governor that restricts how high you rev the motor and how fast you can go. You can change that by changing the computer chip or getting a computer to plug into your car computer and change everything to get more hp, go faster, and rev higher.

computer govener on the older car you can get a wire diagram of the computer harness and clip the govner wire however if you dont know what your doing i wouldnt touch it you could do mor eharm than good check with your local mechanic

The govonor is controlled through your Powertrain Control Module (PCM). Inorder to bypass it you can either have the speed limiter removed using tuning software, or shift into "3" on the automatic transaxle and continue. I've had my 99GT up to 220 KM/H using the 2nd method