I replaced plugs, cap and rotor, and wires at about 80K (I purchased the car at about 70K) the cap and rotor were worn, the plugs could've been replaced, and one of the wires visually didn't pass my inspection as it was discolored in the part that reaches into the plug-hole. I did see a minor (about 5%) increase in milage. I'd rec. changing the cap and rotor and the plugs, and do the wires if you need them. Haynes or Chilton ought to have a spec on the wires (and they are fairly pricey to swap) so if they visually look OK, I'd check the resistance vis a vis the spec before replacing them.
When I did it, it looked to be the original cap, rotor plugs and wires, the fact it only produced a minor increase in gas milage shows how robust the 850's ignition system is...
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