Average speeds of the winners of the Tour de France took a quantum leap upward beginning in 1991, when recombinant erythropoietin, or r-EPO, apparently came into wide use in cycling.

To control for yearly variance caused by course changes and weather, the author averaged the speeds over 14-year periods going backward and forward in time from 1991; the averages are plotted here.

As the graph shows, in the period 1991–2004 the winners’ average speed jumped substantially (8 percent) over the corresponding speed in the period 1977–1990, an increase that cannot be accounted for by improvements in equipment, nutrition or training.

Even after 2004, when stricter controls were put in place, doping seems to have continued unabated: as the graph shows, the 2005 and 2006 Tour races were the fastest ever.

Perhaps the massive disqualification of dopers in the 2007 race finally made a difference: the average speed of that race was below the 1991–2004 average.