So, who will be the first to beat 400 mph (650 km/h)?
Here is the idea, the trick is to have the "rabbits" (the blue X1's) maintining an ideal distance between each others, so that the "fox" (the red X1) can catch the new draft when changing lane. An X2011 is capable of running 490 km/h without draft, and can get up to +580 km/h from a single draft, so I think this is possible.
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EDIT: Imagine a second "fox" drafting behind the first one. 700 km/h, anyone?
With some more rabbits, and a method to get the rabbits up to speed (like having the rabbits drafting behind another set of rabbits) maybe speeds up to 500 mph (800 km/h) is possible? Need to check if the gearbox can be tweaked to do that though...
I also imagine that it would require a lot of practice, because when the fox (running at 800 km/h) catches up with the first set of rabbits (running at 490 km/h) it's like trying to avoid stationary obstacles when running at 310 km/h.