They are often the forgotten men and women of rallying, the tops of their helmets barely visible to spectators through the cars' side windows and their heads down for much of the event, furiously reading pace notes to the drivers alongside them.
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But top-level rallying, which comes to the ACT this weekend with the season-opening Netier Rally of Canberra, is very much a team sport, and co-drivers a crucial element to success.
No-one knows this better than the top-ranked co-driver in the country, Brisbane-born John McCarthy, who is the reigning national champion and sharing the close confines of the Toyota Gazoo Racing Yaris piloted by Canberra's own Harry Bates.
In the yesteryear of rallying, co-drivers were described as navigators because they juggled an assortment of maps, road books and tripmeter readings to guide their drivers from point A to point B.
But the arrival of pace notes - a rolling set of coded instructions issued to the driver via the car's intercom - to the highest levels of rallying has transformed the role and McCarthy, 42, is one of the leading exponents.
During reconnaissance, driver and co-driver usually have a couple of slow, pre-event "passes" over the rally route to develop their pace notes, which are then read back by the co-driver at rattlegun speed during the competition, allowing the driver to commit the car - with a level of near blind faith - to what is ahead.
So if McCarthy calls "six flat out over crest", that's a top gear, 180km/h-plus commitment to the unseen, on loose gravel, with Bates burying the accelerator to the floor.
Thankfully, he very rarely gets it wrong.
"The [driver] commitment [to the pace notes] is absolute; it has to be," McCarthy said.
"At the very core of it, it's fairly simple; Harry uses a one to six system [in the pace notes].
"Sixes are the faster corners and ones are the slowest corners and they do roughly correlate to gears so usually a 'six right' would be a sixth gear right hand corner.
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"Then you add the additional information like 'opens, tightens, half, keep in over crest, don't cut'; these are all the little extra bits and pieces [of information] to help him [Harry Bates] position the car on the road correctly and pick up the throttle at the right time."
McCarthy began rallying in his home state of Queensland with a bunch of mates from high school, co-driving in a 1976 Holden Gemini as a "bit of a laugh".
"I just loved the sport from the word go. But as you step up into the quicker cars, it gets more and more expensive," he said.
"I'd incrementally worked my way up [co-driving] with better drivers and in faster cars ... when I saw that Harry [Bates] was starting out in the Corolla and looking for a regular co-driver so I figured I would give him a call and see if he needed someone.
"And here we are, seven years later, still going strong, much to my surprise."
Bates and McCarthy won the Netier Rally of Canberra last year in convincing fashion and will start car one but expect a much tougher battle when the flag drops on the very strong 56-car field at 6.30am on Saturday.
"There's no doubt that the early stages on Saturday will be absolutely critical," McCarthy said.
"Our plan is to come out swinging."
