Originally Posted by smarkinson

The really top guys would be doing 55+kph. Very fast would be 50kph.

40kph for a 16 year old is not bad and certainly would be faster than most of the people posting here. The UK has a long tradition of time trialling and the shortest distance they usually do is 10 miles (16.1km). A random look at the results for one club ride shows 40kmh would place you in the top third of riders.

But the scary thing is that according to bikecalculator.com you'd need 242 watts to do 40kmh but a staggering 440 watts to do 50kmh. You need 80% more power to go only 25% faster. That's the thing with aerodynamics, you need a lot more power to go a little bit faster. It's also why average speed is not as useful as power meter data when assessing how good a rider is.