What's your favourite gear ratio?

Regarding the 20-40mph jump, surely it's still within the scope of elite trackies if they've dropped back a couple of bike lengths so that they can jump into the slipstream of the rider ahead?

No. They can hit that speed, obviously, but they can't go from 20mph to 40mph in 2s. Whatever the slipstream deducts from the aero drag, it does nothing about the sheer amount of kinetic energy which has to be gained.

Also, the bloke in front has no slipstream to hide in, and your benefit drops off very fast as you get further back, so the bigger your speed differential compared with the bloke you're behind, the shorter the time you have in effective draught. To be in his slipstream from two clear lengths back to running into his back wheel, you have 2s with a speed differential of 2m/s, or around 4mph. So, if you're following 2 clear lengths back at 20mph, he puts the hammer down, uses the banking to max effect and has the ability to put out 28W/kg, he should hit 35mph in 2s. You precisely match his jump and experience zero aerodynamic drag until you hit him, you still need to put out 45W/kg to run up to his wheel in those 2s. It's just not credible, given what we know about the power outputs of elite sprinters over that sort of duration.