I am surprised that you want to preemptively replace the O2 sensor(s). My 1998 is at 188K with original sensors; I suspect that they probably have reached their limit, but still no codes and mpg is only -2 from when it had 150K on them.
New tires typically rob 4mpg for the first 5K miles. A lot of 'cold' engine running will subtract 5-6mpg and coat the O2 sensors with exhaust soot.
Yes, replacing the O2 sensors will increase mpg, but for how long? Enough to make up the investment? Regular here just dropped to $3.90 which is still $1 more than last year, darn speculators.
Klaus
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Always willing to listen, just not able to take direction.
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