Yes, it is under a plastic sliding panel above the engine, under the hood, slightly toward the passenger side if memory serves me.

 

Im still in love with this car Im now averaging 48.5 MPG on over 900 miles. this car is amazing I simply love it! The economy is phenomenal over my previous sunfire because I learn to drive. The car show me how to drive efficiently what to do and what not to do if I want to have good milage. I dont do hypermilling only look at my gauge and try to drive it the way it should for economy. I dont think I could get better than 49 MPG on average but for me it is perfect

 

I hope it continues to work out for you. The best actual mileage tank I got was 46MPG early on. Now getting around 40, and last one 37. Almost 50K miles. Overall, trip B says 40.9 and I never reset it. But my actual mileage was always lower than the trip computer.

 

Really, Doberwiler, I have never seen my computer show higher than actual, usually it is between 0% & 3.5% lower than actual....my mileage logs document this. I know you have heard this before, but a steady decline could be something as simple as tire pressure, increased a/c, increased stop & go even a weak 12V battery...or, at 50K perhaps, you have stopped listening to the car.
I know it really doesn't matter what I say, but, I suspect there is some reason for the decline in mileage, yes it may be the battery but, lets look for horses before we check to see if those are Zebra hoofbeats.