If you call Cummins they will say 900 max, I have hit about 850 pulling a hill at 2100rpm. For cooldown if I have been just running around town I shut it off at 300 or slightly higher, it takes a long time to hit 300 , if towing I wait longer, till about 250. As far as accurate, Cummins only reconmends putting the probe post turbo and they give 900 as the max, my opinion is it is accurate, after all they make the motor.