There are several synonyms for the word "unroadworthy", which refers to a vehicle that is not safe to be driven on the road due to mechanical or structural issues. Some of the synonyms include "unsafe", "unfit", "dangerous", "hazardous", "defective", "dilapidated", "run-down", "worn out", "old", "rundown", "substandard", "faulty" among others. It's important to note that using a vehicle that is unroadworthy is highly risky and can lead to accidents, injuries, or even death. Therefore, regular vehicle maintenance and checks are essential to ensure that your car or truck is safe and roadworthy.