Jim, I'm embarassed to say the time I used to spend considering the types of reinforced light truck tires for a '80 vanagon has been replaced looking at the lines at the local gear head sailing store, APS in this case. Polypropelene covered spectra lines are the kind used in some usscale rescue ropes as well as light lines for sail boats. It's pricey and overkill for a perimeter line but it's nice to know you've got a spare tow line for the car. Used it on one boat, it's a bit stiff. I think the rescue rope must be a different weave it's not as stiff as the sailing brands. Braided polypropelene lines used on some British boats ties/unties easily and doesn't shrink/stretch. Polyester/Dacron comes in all kinds of weave, early Necky boats had the fuzzy 5mm kind, it lasts a long time and doesn't stretch and shrink with water.

Vectran is pretty neat stuff too.

Given the near 1000lbs strength of the cheapest nylon or "weaker" polypropelene in 5mm,3/16" strength really isn't an issue for what your hand can handle.

What with the world political situation I'm worried about all those kayaks using 3/16 and thinner cord for holding the toggles, it wears away too quick.