: An event at the level of this illustration would be pretty much
: run to the legally required maximum 30mph timing, but further up
: the competition chain the times would be manipulated to raise
: the speed, as I did to a couple of these. Fastest I ever
: competed in as a navigator was set to 45mph on some sections --
: on mountain roads in Wales.

Mu example uses the timing method known as Targa Timing. Here is part of what Wikipedia has to say about it:
However, it was eventually banned as it became widely abused - the standard method by which organisers could set very high average speeds in a manner that left no permanent record. https://en.wikipedia.org/wiki/Targa_timing