If memory serves me right, wind resistance increases exponentially as speed increases - in other words, twice the power output does NOT equal twice the speed. So once you reach a certain speed depending on the bike, nearly all your work is spent overcoming wind resistance. As a Clyde and a hybrid rider, I can attest mightily to this factor - I can ride an excersise bike at 15 to 18 mph for nearly an hour with medium resistance, doing 'hills' at intervals but outdoors is a completely different story.

If you look at the Wikipedia article on the UCI Hour Record you'll notice it's only been broken a few times since Eddy Merckx set it in 1972, and even then only by a microscopic amount.

So figure that 30mph is just about a maximum human effort on a typical road bike (and for mere mortals that's only for a few minutes at most).

The difference between say, 21 to 24 mph versus 18 to 21 mph versus your current 15-18 mph is pretty vast indeed, that is, a few mph make a lot of difference work wise.

I'm sure there'll be a lot of computer weenies who'll be able to weigh in with much more detailed accounts and perfectly logical explanations of all this, but for my part I just say get out and ride more, and enjoy it and the rest will all follow

Tom