Matti, the front tyre is a Vittoria Mota 26 x 2.5". It is a bit of a monster, but never seems to run out of grip. The rear is a Schwalbe Nobby Nick 26 x 2.25" which seems to pair up nicely with the Mota at the front.
The frame was built with horizontal rear dropouts. I'm using a 36 tooth chainring with a White Industries 18 tooth freewheel for the classic 2:1 ratio. The rear hub is a DMR singlespeed hub and the Salsa QR is helped by a BMX chaintug. Nothing particularly high tech, but it works.