Depends on where you drive it. In stop and go traffic regular drive. On the Highway overdrive will save you a lot of money. you can see the rpm's falling when using overdrive. What's the downside, the Taurus is notorious for transmission problems. I'd recommend at least once a year changing the oil and filter(transmission). you will find a lot of filing in the bottom of the transmission and a magnet that collects them, keeps them from re circulating through the oil and filter. I probably use the overdrive too much, because I do this regular maintenance myself. The gas saved in overdrive is considerable. Downside is too much changing of gears in heavy traffic, making wear and tear on the transmission. I just feel like the maintenance wit ll keep the problem away.
Besides I'm enough of a handy man I can with a digital camera fix almost anything.