That doesn't seem likely to me, because she doesn't pedal when she's being towed at the beginning. She only begins to pedal at around 0:30 in the video. So I conclude that there's a freewheel.

Someone might suggest that the extraordinarily high gear ratio means that the initial acceleration involves just a tiny foot movement, but I think it would be severely disproportionate to what we see -- one of the interviews referred to the bike moving 128 feet per pedal revolution (quoted elsewhere in the comment thread). Clearly the bike has been towed much more than 128 feet before a pedal revolution has taken place, right?

I would think a fixed gear under the circumstances would be very dangerous for various reasons, including the extremely long deceleration time at this speed and the extreme hazard of applying friction brakes. A fixed gear would mean that giving up the attempt (or even just ending it normally) requires a long period of continued effort to avoid injury and a very likely loss of balance.

I wanted to contrast this with velodrome speeds by pointing out that this is over 3× the unpaced velodrome speed record and that the unpaced velodrome record allows the cyclist much more overall control of the situation. On the other hand, I don't know for sure whether the motor-paced velodrome record (which is about 2/3 of Mueller-Korenek's new outdoor record) used a fixed gear.

It's fixed gear. It doesn't seem like she's pedalling at the start because the gear ratio is soo large, so large in fact the bike can't even be pedalled at speeds less than 15mph.

But if so, shouldn't there have been several pedal revolutions at the start? Surely she was towed much more than several hundred feet?

I found a new piece of evidence that it's a fixed gear: https://www.youtube.com/watch?v=cZhrqKrljig&t=265

The pedals continue to be pushed forward when the rear wheel is rotating freely. With a freewheel, this shouldn't happen (the pedals can drive the rear wheel when they're moving forward but not backward, while the wheel can drive the pedals backward but not forward).

But I'm still confused about the lack of visible pedal movement during the initial towing part of the record video. It seems like she was towed more than far enough that the pedals should have needed to move visibly with a fixed gear.