How liable are you for the misconduct of travel companions?

Obviously this varies between jurisdictions, but I don't think any jurisdiction would consider you an accomplice for something you had no knowledge of.

However, some jurisdictions make it a (completely separate) crime to not notify the authorities of certain planned or commited crimes that you are aware of, and of course depending on the nature of the crime the police might believe that anyone travelling together with the perpetrator must have been actively involved.

So while you may not actually have done anything illegal, you're quite likely to be considered a suspect, which (in case of a serious crime) can cause you all kinds of trouble, and with a corrupt or incompetent police force, can land you in prison regardless.

Without specifying a jurisdiction, every answer to this question would be "maybe, or maybe not". There's a legal term for this: being 'accessory' to the crime ('aiding and abetting' in some countries / situations). It also depends the severity of crimes and usually, the more a severe a crime, the more likely it is that you can be charged as being an accessory. There can also be a distinction between 'accessories before/during an act', i.e., you were around or knew beforehand that your friend was going to commit a crime as opposed to 'accessory after the fact', where you may find out later that your friend committed a crime.

In all, it's variable but the rule of thumb is that more often than not applies to 'serious' crimes rather than offences such as speeding. (In which case it would bring up the question - why are you hanging out with a murderer / drug dealer / pimp anyway?)

Wikipedia has a summary of accessory laws in some countries of the world that you might be interested in.