I keep meaning to do a resistance test on old wires compared to new wires, but I still haven't bought the new wires and I have no idea how old the wires in my car are so it wouldn't be a seriously definitive test.
I can't imagine that they seriously degrade as much as wire suppliers would have us believe. Wouldn't wiring in houses be constantly degrading as well and need replacing much sooner than 40+ years?
Most spark plug wires are NOT really wires! The insides have carbon and some fibers around them then the silicon. the more you move them or they are vibrated, the more the carbon can separate and resistance goes up. That's why performance wires are spiral core which as the name implies has a stainless steel wire wrapped in a spiral then several layers of silicon.
Kevin A Thornton
For Speed Equipment, Nitrous Express firstname.lastname@example.org
Plug wires are carbon core because they are resister wires, if they were lower resistance, like pure copper, you would hear your engine running everytime you turned on anything electronic, like your radio, that is part of why older motorcycles still used copper stranded plug wires.
it is commen for the insulation to be the first part to wear out, it gets cracks and you get arcing, that is why you can tell when you need new plug wires by when your car runs poorly in the rain, and if you look at your running engine at night, and see arcing, you need new wires.
if your house wiring was exposed to heat, oil, water and grit, you might have to replace it every few years because the insulation would degrade to much.
I changed most of my ignition components it looks like it improved my mileage a bit. I changed the spark plugs, spark plug wires, distributor cap, distributor rotor. I hate bosch stuff, my plugs and wires are now NGK, it's much better quality than bosch.