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Often people choose to overclock their processors which is really only achievable from the bios or UEFI, or in Windows.
It has taken a long time to find the answer but when overclocking the true clock speed has never been shown in nix.
Often the command used which shows the various stepping speeds a processor will go through is:
To strip the output down to just the speed use:
cat /proc/cpuinfo | grep -i mh
This is great but recently on a I5 4670K series CPU the clock speed was changed from 3.4GHZ to 4.0GHZ.
sudo dmidecode --type Processor
Now the correct speed shows after overclocking.