Check this out...
This is a chart showing overall gas mileage over time for my Mirage DE. Focus on the decreasing leg of the chart where the mileage goes from 38 to 36.5 ish. This leg coincides EXACTLY with oil changes - EXACTLY !!!
Here are the stats:
Date Mileage Gallons Tank MPG
10/2/2014 3007 5.038 44.86
10/12/2014 3150 4.420
32.35
10/28/2014 3350 4.841 41.31
11/19/2014 3496 5.057
28.87
12/11/2014 3682 5.474
33.98
12/24/2014 3830 4.699
31.50
1/19/2015 3968 3.315 41.63
3/10/2015 4131 5.095
31.99
3/27/2015 4282 4.656
32.43
4/23/2015 4525 7.024
34.60
5/12/2015 4687 4.788
33.83
6/3/2015 4968 6.926 40.57
Those are some seriously poor MPGs, some during winter, but I generally don't drive the Mirage in the winter unless the roads are dry, so mainly during better weather. I didn't drive far, averaging 10 miles / day during the time period shown above.
The thing that has me wondering is why does the mpg show such a strong correlation to the oil changes? The oil was changed on 10/2/14 and 6/3/15. After the 6/3/15 oil change the mpgs have steadily risen. I remember that the 10/2/14 oil change was diy using the correct weight oil but not synthetic and using a non-OEM filter. The 6/3/15 oil change was done at the dealership.
So, to the OP, maybe this helps.