The slowest Tour de France in history was the 13th edition between 29th June and 27 July in 1919. The race included 5560 km, divided into 15 stages. This was the second longest Tour de France in history.
Due to the recent war times (World War I between 1914 and 1918) most of the riders had not enough possibilities to train. This is one of the reason why mostly the older riders with more racing routine dominated the competition.
On the other hand, the international elite peloton suffered some important losses: three of the former Tour de France winners, Lucien Petit-Breton (winner of 1907 and 1908), Francois Faber (1909) and Octave Lapize (1910) died during the war.
Also, the French road system suffered severe damages, especially in the North-Eastern part of the country.
No wonder, that this race had the slowest average speed in the history: 24,1 km/ h
The 13th Tour de France was won by Firmin Lambot, who also won the Tour de France 1922.