Want to pick your brain on this...
If someone were to upgrade the chassis-to-negative terminal wire for their battery, could they rely on the chassis for ground to avoid running a second large conductor?
It's possible to improve conduction to the chassis tie in point, (bigger cable) however, as with any electronics, there are variables. So, one would have to evaluate the conditions and use such as the distance between the chassis connection, and distance to the device to run. The real issue is that the chassis is not "pure copper" so conduction will never be as good as a
substantial "pure copper" conductor. But we are talking about HUGE current draws to make a substantial impact. In most all cases, chassis ground is sufficient to run most accessories with relatively low current draw such as one or two sets of lights. But when it comes to high current, the chassis, which is generally steel, is not a good conductor and the fault is that it will drop some voltage as a result. For this reason, most high current devices require a second conductor equal in size as the primary conductor. Even a winch uses two conductors. It's not the cable that's the issue, it's the steel "conductor"; AKA the chassis.
There are conditions where the chassis can be a sufficient conductor with the right architecture. Take for instance, the starter motor; a high current device, but only one conductor. The reason this works "ok" is because the starter has a ground through the engine ground point, not the actual chassis. The key here is the short distance between the motor ground and the ground plane it is mounted to, which is the engine, a substantial amount of steel or aluminum. Essentially, it has a copper conductor right up to where it ties in with the negative cable.
In other cases, where voltage is not critical for the device, a chassis ground can be used with loss if the device is intolerant to the drop. An example might be a DC heater, it will lose some power, but it will still heat up. Not as efficiently, but sufficient for its intended use. (Though I would run two conductors to avoid any power loss.)
Hope this helps.
