Particulary in July. Was there for two weeks in 2013. First week was chilly but mainly wet with sunny periods - lost of low cloud.

Second week when we had the bikes we had sunshine which brought it's own fun with "sea breezes" - I would advise not riding the 35 solo if there has been recent heavy rain as the river crossings can be hairy.

If the weather is against you as well the Westerfjords can be dodgy as well. Even though we had blazing sunshine we also had to admit defeat with two days of sea fog giving zero visibility, so on the gravel sections on route 60 it was.... interesting. But I would ride it if you can they are creating more tourist friendly sections so the fun stuff will be gone soon.

We ended up catching the ferry over to Stykkisholmur on the Snæfellsnes peninsula and spent a couple of days riding there - great gravel roads as well as good tarmac and worth hunting out the Kaffe Sif in Rif http://www.snaefellsjokull.com/hellissandur-rif

Also consider getting hold of some local maps when you arrive particulary if you hit the Highlands. If the weather is going to be bad I can recommenced the Kaldidalur (550) route from Thingveller to Husafell for a great days ride, when you get to the end you have t junction with a large campsite and petrol at Husafell on the left but a great little basic campsite at http://www.fljotstunga.is/#!the-farm/czq2 on the right

Pack and prepare for Maritime European early spring weather and you will in general be fine. We took two season bags with thick silk liners and lightweight primaloft thermal gear - which was great for the first week but ended up in t-shirts and shorts for the second