I do effortless(=no transpiration) 30 to 35km/m on flat grounds with no headwind sustained. Where 'sustained' means something like 10 to 15 minutes max, since after that I either arrived at my location, or on longer tours I have to slow down many times for other traffic, pedestrians, traffic lights, etc.

On a three speed 28" from ca. 1985 btw., weighing 12kg with side dynamo, lights, mudguards, rack, and a wirebasket fixed with a bunch of cable ties on top of that(for my back pack).