Temp just dropped significantly around here and my MPG went from 40 to 37. If you can rule out the weather then I'd suspect maybe they put in heavier oil than before.

 

Yes. You may want to confirm that the dealership put in 0W-20.
Otherwise they may have put in whatever is in the bulk tank which is likely 5W-30.

 

Two weeks back my Tank Average Mileage dropped from 37 to 32, which coincided directly with the change in temperature (otherwise, my driving pattern is identical every refill). I've determined this is because of my short trips without the car having time to heat up, which means the oil is coagulated (esp if anything other than 0W-20) and the gas engine is always on even when idling to heat up the vehicle (even worse with ECO off). My solution is to ensure ECO is on (note, it turns off when you use the front defroster) which allows the Synergy drive to go to E mode more frequently while trying to maintain interior cabin temperature, and turn off the heater completely at stops or on short trips (which is conveniently on the steering wheel controls). This has helped go from 32 to 35 mpg week over week (which I have nearly identical driving patterns each week and it was actually colder this week). I am using 5W-20 which is another opportunity for improvement, espically in cold weather with short drives, which is what I am doing primarily. I wanted 0W-20, but forgot to specify this on my first oil change (beginner's mistake).

 

Did the service have anything to do with the brakes. Our dealer tends to tighten up the parking brake to the point that it's dragging slightly, even when released.