My gas mileage has really tanked since I got my '00. I've had regular maintenance and a major tune-up just a couple of months ago. No problems were found. I was wanting to replace O2 sensors but the mechanic didn't want to since the service engine light was not on. My car is coming up on 100K miles and I didn't think it would hurt to replace them. Anyone else done this just as "preventative maintenance?" Results?