Wire gauge selection is not just how much current the wire can safely carry. Most of time with high current loads it's more of voltage drop over the wire. You can calculate the voltage drop if you know the current and length of the wire.

Use a table or calculator to get the resistance of your wire.

http://hyperphysics.phy-astr.gsu.edu...es/wirega.html

Here you will see a 1000 feet of 16 AWG wire is 4.016 ohms. So if you use 16 AWG for your fuel pump and it's 10 foot from the RPM relay to the pump and 10 foot from the pump to the inertia switch then that 20 foot of wire will be 0.08032 ohms. Now use ohms law "E = I x R to get the voltage drop at 7 amps.

16 AWG drop would be 0.56224 volts.

I think there is more than 20 foot of wire but you get the idea.