If you are a confident cyclist, slipstreaming is a positive gas! I have (a few times, only) "drafted" behind a large vehicle--truck/bus--as it pulled away from a traffic light. I rode them suckers up to bout 50 and for perhaps 1/2 mile when I either ran out of gear or nerve, depending on the situation. It is very heady, but I do not recommend it for any but the steel-nerved. And if the wrong person is watching, you CAN get a ticket. Following too closely, wreckless driving, something like that.

In another situation, I spent much/most of my (solitary) double-century riding a line. My group finished in just over 14 hours. But we certainly would not have been so chipper at the end, nor would we likely have finished so fast if we had been riding "normal" group ride style, side by side.

In short, at slower cycling speeds, one's energy is used mostly to overcome friction within the bike, and some gravity. As one approaches about 22 miles per hour, the energy drain shifts to wind resistance and increases either geometrically or exponentially, I fergit my physics. This is also why many racing cyclists tend to be shorter--less body to resist the wind, and easier to tuck in behind another cyclist, regardless of their size.

Hope I didn't bore you.

Saul