First, Google is wrong when it claims that 36 and 52 are connected. Take the first option, the second one is just dangerous.
You can see the correct setup here: vegasja.vegagerdin.is/… Google might show the correct numbers if you zoom in. Road 550 is a no-go road to most rental companies in a 2WD.Edited: 11:38 am, May 17, 2016
Route 52 (Uxahryggjavegur) is fairly rough gravel. While it is not an F-road, it will be slow driving, and not recommended for compact cars or inexperienced drivers. (Some rental agencies may also not allow you to drive it.) It will take you up to the edge of the highlands.
Depending on your itinerary, you may want to travel via Thingvellir National Park by following 36 up past the park and then on to Route 1 (by staying on 36).
Rikki and Kevin, thanks for pointing out the dangers on route 550 and 52.
I will not go that way. Lucky I ask for advise.
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