"Referred to" basically means (in this context) "talked about". If you were having a conversation with someone and they were to "refer" to the sky, they would just be talking about the sky. It's also used to direct someone's attention towards a certain object - for example, being instructed to "refer to page 10 of the document" would mean that you were focusing on that specific page - in this case, focusing on this specific Prius. In other contexts it can also mean to direct a person towards another person (if you were, for example, to go to a doctor, they might "refer" you to someone better to help you, such as a specialist). Not here, though. "Labouring its way", with or without the "u" in "laboring" (British English or American English) means it had to work very hard to get to that speed. Since a "labo(u)rer" is a person who does manual work, usually pretty physical work that takes a lot of effort, maybe you can compare the phrases that way.