How do I set the clock speed fuses on an ATtiny85 when using an Arduino as a programmer?
I'm using Arduino UNO + ArduinoISP successfully.

Add -U flags to your avrdude command to set any or all of the three ATtiny fuses.

avrdude -p attiny85 -P com8 -c stk500v1 -b 19200 -U lfuse:w:0x6f:m -U flash:w:main.hex


The clock selection is done in bits[3:0] on the third fuse ('Fuse Low Byte'). Set them as follows to make use of an external crystal (of 8MHz or faster):

-U lfuse:w:0x6f:m


Its definition (I infer) must be something like: [Fuse Low Byte]:[write]:[hex value]:[set manually]

The default value for the four high bits of this byte are 0110, so leave the 6 in 0x6f as it is, and only change the second digit, the f (its default value is 2).

NB: If your processes take longer or shorter than you expect, check your clock prescaler and your definition of F_ CPU.