Just had 90K service and mpg is down??

I just had my 96 3.8 ES in the shop for it's 90,000 mile service. They did a major tune up amoung other things. I reset the mpg display and now it shows around 14 mpg! The shop says they only use OEM parts and everything (timing, air/gas mix) is set my the ECU. Do I need to reset the ECU or something?

Looks like the mpg is going back up. The monitor read 17.4 avg mpg last night.

Did you do any highway driving?