
http://www.race-dezert.com/forum/threads/changes-for-the-2015-mint-400.120922/page-3#post-1660561
The 6 hour cut off is the end of the second lap."New Longer Mint 400 Race Course - All Classes Race Every Lap
The new 126-mile Mint 400 race loop is designed to allow all twenty classes of race vehicles to race a full three laps of the ultra-technical race course - provided they can make the six hour cut-off time. The loop is longer and more technical than previous years and will allow limited classes to get more seat time as they attempt to finish the notoriously brutal race."
In order to do the full 3 laps in 6 hours you have to average 63 mph. Wonder when the cutoff is to start the 3rd lap?