Thanks
@tommyk90; that was a good explanation.
To elaborate - what we really care about is voltage drop. we care about that because most amps will produce less power with less supply voltage. voltage drop can occur on both power wire and ground wire. as mentioned, we are trying to complete a circuit.
"load a ground" means to pull current through it, so voltage drop can be measured. a standard DMM is not accurate enough to measure ground resistance (inaccurate at low resistances). there are separate tools for that, or you can just pull a known current and measure voltage drop and calculate it.
There are no "hard and fast" rules, but if your expected draw from the front is less than 200A, you are probably fine with using good steel (not necessarily the body). if you start to run parallel sets and multiple alts, run dedicated grounds.
It certainly won't hurt to run dedicated grounds, and in reality, resistance will be less since you have continuous copper from back to front and not separate pieces of welded steel.
The more I think about it, the more I think that dedicated grounds are a good idea for anyone... lowering resistance is the goal, and each connection contributes for the majority of resistance. More importantly is the resistivity of metals...
The Resistivity of Copper is 1.712
The Resistivity of Steel is 11.8
The Resistivity of Aluminum is 2.709
Resistivity is 10^(-8) ohm-m @25 deg C
Lower resistivity means better conductance.
Taken from "CRC Handbook of Chemistry and Physics, 57th Edition, 1976-1977, CRC Press, p. F167-168; CRC Handbook of Chemistry and Physics, 90th Edition, 2009-2010, CRC Press, p. 12-14, p. 15-37"