Hi

I have a 1994 850 turbo (170,000). Fairly recently replaced the fuel pump, o2 sensors and alternator. I have always gotten 19 mpg city and highway(according to the onboard computer readout in the average mileage setting). Today I started my car and noticed the average mileage readout to be 14.5 mpg. I was shocked and it kept going lower as I drove around town. I decided to drive on the highway and see what would happen. To my surprise my mileage jumped up all the way to 29 mpg!! After I came back into town after a short while the mileage began to drop again fairly fast now down to 17 mpg?
I just changed my oil to synthetic for the first time, though I don't think that would have anything to do with it?

Also the car is driving just fine. No stalling, rough idle, nothing weird at all, not tons of smoke or weird smells, nothing at all out of the ordinary except the weird mileage!

If you have any advice or ideas to what may be causing this please respond to this posting.
thanks in advance
ryan