And by the same logic, a 1000 shouldn’t be a substitute for a 600 due to the lower average speed required to get to 600.
Time limits for rides longer than 600km aren't completely linear. ACP has some convoluted rules on this:-
http://www.audax-club-parisien.com/EN/322%20-%20Rules%20of%20BRM%20Worldwide.htmlalthough I'm not sure AUK follows them for its own BR rides (or even BRM rides in the UK). (LEL certainly doesn't, but that's homologated by LRM not ACP.)
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Article 10 : Overall time limits vary for each brevet according to the distance. These are: (in hours and minutes, HH:MM) 13:30 for 200 KM, 20:00 for 300 KM, 27:00 for 400 KM, 40:00 for 600 KM, and 75:00 for 1000 KM.
Additionally, riders must arrive at each checkpoint between the opening and closing time for the checkpoint and calculated as follows:
Opening: 34 km / h (km 1 to 200); 32 km / h (km 201 to 400); 30 km / h (km 401 to 600); 28 km / h (km 601 to 1000); commercial rounded by the minute.
Closing: 1 hour + 20 km / h (km 1 to 60); 15 km / h (km 61 to 600); 11.428 km / h (km 601 to 1000); commercial rounded by the minute.
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This actually makes the closing time of a checkpoint at 400km of a BRM 1000km EARLIER than you'd get a full 400km BRM ride:-
* BRM 400 gives you 27:00 for the entire ride.
* 400km at 15kph = 26:40, 20 minutes less
On PBP, you don't get 45h to get to Brest, I think it's something around 43h.