Cd is a relative measure. Following in the low pressure area behind a truck (or other rider) helps you go faster because there's lower pressure and hence less air to move out of the way.
Surely it's more a case of the air moving at about the same speed as the truck, so you don't have to move it yourself?
Arse. My previous reply was lost by an interweb failure so I'll, try again.
I'd hate to get into a technical discussion with you Kim 'cos I'd invariably lose but, AIUI, any moving object creates an area of low pressure directly behind it because it's moved the air out of the way. To take advantage you have to be in the middle of the moving object and very close to it.
My 1st sentence stands 
.
Ok - let's use my long-distant Aeronautical Engineering degree to lay this out (and probably mis-remember stuff and lay myself open to ridicule).
The form drag of a body moving through a fluid is given by:
F = 1/2 r v
2CdA
Where r = air density (I can't do rho here!)
v = velocity relative to the object
Cd is the drag coefficient
A is the cross-section or reference area of the body.
Therefore to reduce drag we can:
- Reduce the relative velocity of the fluid
- Reduce reference area
- Reduce the density of the fluid
- Reduce Cd
Drafting behind another body achieves 1 above - the air behind the body (cyclist, lorry, car...) is entrained behind it and is being dragged forward slightly and therefore
relative to the cyclist is reduced.
Drafting
might reduce density, but not much at all - it's really all about reducing relative fluid velocity.
An aero position achieves 2 above - the frontal area is reduced. The elongated shape of an aero rider also reduces the Cd.
Cycling at altitude reduces the density thus achieving 3 - but then we all know what happens when you try to exercise at altitude...
Wearing slippery clothes/aero helmets, deep rimmed wheels achieves 4 above by - well - being slippery, and reducing eddies around the rims. Also fairing other stuff on the bike achieves this.
Footnote
There is, in fact another form of drag, namely
induced drag - this is proportional to the lift produced by an aircraft's wings. What's that got to do with bikes? Well... in a crosswind, the bike and rider will produce 'lift' to one side - therefore a crosswind will produce some induced drag - and especially with those lovely deep rimmed wheels 'lifting' hard sideways. However, the advantage of the reduced Cd deep wheels give will outweigh the much lower induced drag.
Now back to the main topic of this thread.
