Okay, timelines...
So we have pings from Karen's mobile phone 40km from her home (near Gisborne) "a few hours" after she reportedly went missing at 10am.
We don't know what time the pings were detected from BR's mobile phone near Diggers Rest.
We do know that his phone was switched off for two hours on the day she went missing. We don't know what time this was.
From this information it sounds as if he may have been disposing of her body during the day instead of the night before (plus we don't know who last saw Karen besides BR and when).
So if he was transporting and disposing of her body during the day, that's pretty risky IMO. The risk of being pulled over by police (though if he's driving carefully and not speeding that's unlikely). The risk of the car being caught on CCTV or possibly even speed cameras on the Calder Hwy. The risk of being noticed by a passerby when he was disposing of her body in broad daylight. He would want to spend as minimal amount of time doing that as possible. Otherwise the risk of getting seen increases. It would also have to be a very isolated area where he has some knowledge of it being quiet with not much through traffic.
Would it be smarter of him to drive on the Calder Hwy and main roads versus back roads? Back roads - may take longer possibly. Maybe not. Less likelihood of cameras being around on back roads. Calder Hwy and main roads - quicker possibly. Less time transporting a dead body. But more likely to be seen and noticed by: (a) cameras, (b) police for a routine traffic stop, (c) other motorists.
Thinking out loud here. Any input would be gratefully received sleuthers.