I am trying to set the gpu fan to a fixed speed but with "-tt 70 -fanmin 80" but I get an error "Failed to set new fan speed, check if miner has root access!".
Has this already been discussed? I could not find an advise to fix this.

afaik  ethdcrminer64 process runs under the user "miner" which has sudo root rights:

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL
miner         ALL = (ALL) NOPASSWD: ALL

Is this not enough?