How thick of a rope would you need for pulling cars or trucks? All I know about strength of Nylon is to check the charts for our lifting straps at work.
1" nylon 3 strand twisted would give you a SWL of about 2,800 lbs. and a breaking strength of about 14,000. But, as I said earlier, splicing an eye or putting a knot in it reduces that by about 1/2, and nylon loses about 25% of its strength when wet (soaking wet, not just rained on), so you might want to go a bit heavier, say 1.25" or 1.5", depending on the loads you expect to put on it.
However you do it, just remember to use common sense safety precautions such as not allowing anyone to be in-line (and for 45 degrees either side) with the rope while under load, using proper attachment hardware and attachment points (it really sucks when you pull on a bumper and it comes through your back window), making sure the vehicle you're pulling is in neutral with the brakes off (amazing how simple that is to forget), wheels pointed in the direction you want it to move, etc. Also, understand the difference between a static load, dynamic load, and a rolling load.
Static load is a load that's not moving, just hanging or just under tension.
Dynamic load is a load that you're actually pulling on and trying to move, or is moving.
Rolling load is the rolling resistance of the vehicle you're moving, and differs from trying to drag that same weight without the benefit of wheels (or flat tires, high centered, etc.) and will be easier to move, but harder to stop (due to mass and inertia). As an example, one guy in decent shape can get a 5,000lb. vehicle rolling pretty easy on level ground with four good tires, the brakes off and the gears in neutral. Ten such guys would have a helluva time moving that same vehicle 6" with the brakes on, flat tires, or the tranny in park.
