Generally we find that the Tachs are off by 150-200... so the actually idle is closer to 750 Give or take a few.. It sounds like you are on the money for idle..
For the O2, for the 94,95,96 model years they all use the AFS 75 Sensor.. But they seem to drop off in performance after 60,000 miles.. I have seen cars go from 24-25 in mixed driving to as low as 18 in the same driving situation..
Example in the city would be like going from 20-21 to 15..
More than most if the car is fairly well tuned and the mileage is still stagnant then it may be due to the O2.. I have seen them just about 90% dead, and the PCM won't thow a code till its almost time to throw the paddles at the O2..
I do not like the Bosch at all... But if it was a free sensor to you than you may try it, as it may function ok for a while and improve your mileage.... But I do recommend that you get an AC Delco or Denso unit when the $$ allow...
The Factory rating for a 95 Bonneville SE with the 3.06 Ratio is 19 City and 29 Hwy... I do 17 to 21 and 28-32 depending on my driving habbits
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