In reply to Jack Gillespie: I have driven the F35/Kjölur two years ago just after it opened for the season and also done quite a bit of cycle touring in other countries.
Going south, the first half was rather smooth. It would be fine with something like 28mm tires. I was seriously considering asking some of the cyclist we overtook in the car to swap with me for a while.
The southern half (after the hot springs and campground which name I cannot remember right now) had much looser and bigger gravel and quite steep hills. I'd rather have had something like at least 40mm tires there. Not sure if a Galaxy will accomodate these.
Of course conditions will also vary from year to year and throughout the season as they tend to plane the roads, too.
Enjoy Iceland, absolutely stunning country.
Mike