I bought this used at 34000 miles.  I now have 49000 miles and it only
gets 15 miles per gallon in the city and not much better on the
highway.  It runs great, idles great, no smoke etc.  I just replaced
the tires 3 months ago and this has not helped.

I ran my finger around the inner portion of the exhaust pipe and there
was black soot.

Would just replacing the O2 sensor fix this or should I replace the
plugs, cap and wires as well?

I only use Chevron gas.

Any help would be appreciated.

Thanks so much!