I average about 15mph/24km/h daily and once a year I do a max run, and this year it came in at 29km/h, thats about 18mph. it was with a backpack and a fendered bike, with empty (quite big) saddle bag. good tires.

If i were to do this on my skinny tired naked bike i might get up to 30-32km/h. but i could only do this maybe twice a month i feel.