If you choose to start driving south, you will be driving on the ring road until you arrive to road 35 on your left. That’s 55km (31miles) from Reykjavik and it takes you around 45 minutes before you have to turn left. Geysir is on route 35. Route 35 is one of the interior roads of Iceland. The road is divided into two main sections. The first part is called Biskupstungnabraut.
You can also drive through Thingvellir the National Park, Lyngdalsheidi, it is about an hour drive to Geysir from Mosfellsbæ. Find your way to the town of Laugarvatn. From there, follow route 37 which turns into route 35 and leads to Geysir. Don’t worry, there will be road signs.