My 92 Civic has very old spark plug wires, probably original, but their resistance is in-spec (under 20k ohms), they have no external damage, and the vehicle has no misfire or other symptoms associated with bad wires, aside from having worse gas mileage than it used to. Is there any reason to replace them? Would doing so perhaps get a more optimal spark timing or stronger spark, more-complete combustion, etc.? Or are those claims just made up to sell wires?
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Mostly replacement is preventative maintenance. Eventually they'll get cracks that moisture can get into, etc. If they test good, they're probably fine for the moment, however, at their age they could develop problems any time.
On my '91 Toyota, I've been through several sets now, and have to say that in my case, OEM is the best. I've tried other brands (including "performance" brands) and have never seen any measurable improvement in performance. I have, however, had a lot of early failures with those supposedly better (and dramatically more expensive) performance wires.
If it ain't broke don't fix it. If your wires already check out and you don't have misfires it is probably not worth replacing them just yet.
You could possibly get better performance out of better coils and plugs but even those numbers are pretty minimal. Best thing I can recommend for fuel economy is to use seafoam in the engine and fuel and then change the spark plugs soon afterwards.