It was part of the discussion at Lonely Planet forum.
For those who rent a car from Keflavik it might be a good idea to dive not straight to Reykjavik but make 70 km “detour”. Depending on your desire to check everything you meet (and weather) it might take extra 1-3 hours. But you won’t regret it – as you’ll see much of the Icelandic landscapes right away from parking lot (except iceberg lagoon, Geysir and glaciers – all other stuff included). The drive is easy although much of the road is unpaved (but allowed for all types of rented cars).
From LP discussion
Roads to go 41 > 44 > 425 > 427 > 42 > 41
Map instructions – click on pins to see driving instructions.
View Keflavik – Reykjavik drive in a larger map