Werner and ZT,
I like your comparisons, because they give me an excuse for my swimming. I won't consider jogging, because I am too arthritic to do that, but I don't think I can hold a pace of 30 km/hr on a bike. I would say that 20 km/hr on a flat stretch is more realistic for me.
Of course, all of these are "rules of thumb" with lots of room for exceptions. My bike is not a racing bike and it has large tires. But if 30 km/hr on a bike is the price in conditioning that needs to be paid in order to swim a 30 min mile, then I have a good explanation as to why I am nowhere near being able to do this!