A mate has made an observation regarding pre-packaged amp power cables from car audio stores. Him and I have seen cables with smaller gauge earth than the power cable.
But due to basic laws doesn't current in = current out? In that case would the earth cables be restricting power?
anyone fill me in if i'm wrong?

thanks

Not if the current is absorbed, so to speak.

If you put a resistor in a circuit, and the voltage is the same coming out the current will be less.
Speakers are resistors.

when you say earth wire, do you mean the ground wire? That's what I think, but I'm not sure.

This is correct. Resistance of a cylinderical wire is defined as resitivity * length/area

Not if the current is absorbed, so to speak.

This is true, in a sense. However, the grounds to the speaker isn't chassis ground like the ground to the power amplifier. The speaker ground is just the differential output of the amplifier.

In the end, you have the main current input being split up into current driving the speakers and current being used to power the amplifier. The current to the amplifier itself is returned to chassis ground, however the current to the speakers is returned to the amplifier. Therefore, the current coming back in must go out to chassis ground. So in this case current isn't being spent somewhere else.

Ground wires tend to be considerably shorter runs, as well.

Exactly. you can get away with a smaller gauge wire if the run is shorter. In my car the ground cable is less than 3 feet while the main power run is closer to 15 feet as i had to snake it around to hide it. I used 4ga for both but could have used smaller for the ground if I wanted to.