Teensy 3.1 no overclock option for 168 MHz
Using the latest Teensyduino distribution as of this writing, 1.25, you have to uncomment the higher overclocking options in the boards.txt file, located here in my install-
/hardware/teensy/avr/boards.txt

The options look like this, just remove the # symbol-  

#uncomment these if you want to try faster overclocking
#teensy31.menu.speed.168opt=168 MHz optimized (overclock)
#teensy31.menu.speed.144opt=144 MHz optimized (overclock)
#teensy31.menu.speed.120opt=120 MHz optimized (overclock)`