Easy "Centrifugal Convoy Adjustment System" Achievement
The achievement "Centrifugal Convoy Adjustment System" wants you to pass the same portal pair 15 times without interrupting the cycle by passing a different portal. The easiest chamber to do so is chamber 42. Just hit "drive" and wait for your vehicle to pass the portal 15 times. Be aware of this achievement won't pop up, so you have to count carefully and leave the level afterwards to get the achievement.