You could drive 5 miles over the posted speed limit when you need too.
However driving 5 miles over the posted speed limit would be considered speeding and you could get pulled over if a cop is out watching and catches you going over 5 miles per hour over the speed limit.
If it's your first offense though the cop may decide to just give you a warning if you're nice to them and apologize for it.
However you could still get a speeding ticket if the cop decides to write you a speeding ticket.
So always watch your speed and if someone else is speeding and you need to speed up to get out of their way then do so just for a short while and if possible pull over to the shoulder and allow the speeding vehicle to get ahead of you.
It would be better for them to get the ticket than you.
I was pulled over before for going 5 miles over the speed limit and the cop just gave me a warning but he said next time it would be a ticket.