We tried some 25 mm Continental GP4000S tires and thought they were very harsh. Then we tried some 28mm Continental Gatorskins and thought they were better. Then we tried some 28mm Schwalbe Ultremo ZX tires. I'm not exaggerating when I write that within 50 feet we both said "Wow... these are nice!"

The Schwalbes are so smooth that they transformed the ride quality of our Cannondale RT1000. It feels like they roll fast, too.

A couple of other tandem teams we know use them and really like them.

We ride on mostly smooth, clean roads, but sometimes I don't notice the occasional broken glass or small pothole. So far (knocking furiously on wood) we have not had a flat in about 200 miles.