Bring the traditional outdoors and cycling clothes such as padded cycling shorts, Lycra pants, cycling jerseys but leave the cotton T-shirts and jeans at home. The cotton takes a long time to dry and will not keep you warm when wet with sweat or rain. You'll also need rainproof clothes. We highly recommend choosing high quality Gore-Tex clothes tailored for cycling that allows your sweat to escape. You'll also need a warm fleece sweater or jacket and outdoors underwear and socks to keep you warm when you're not cycling. Something to keep your head warm should also be brought along and gloves. Helmets are mandatory in Iceland for everyone under the age of 16.
A lot of dust can be thrown up from the ground because of traffic and erosion so cycling glasses could prove useful, clear or yellow are great on the not so sunny days. Wading shoes such as neoprene reef shoes or light tennis shoes are quite useful when you have to wade rivers. You will not need them on Route 1 but if you choose the more interesting jeep roads or go inland you'll find unbridged rivers with freezing cold water and slippery stones that have to he waded across, sometimes carrying the fully loaded bike to protect the sensitive bearings. Some rivers can be dangerous to cross at times so take care.
You might like to take a little hike after a long day on the cycle but also expect to do some walking up hills and in sandy areas, so good walking- or bike and hike shoes are recommended. Rain covers or gaitors to keep the shoes dry in rain can be quite useful if you get rainy weather so bring them along if you own them.