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A lot of things can affect your mileage, like rain, temperature, wind, tire wear, etc. That is why many people here will tell you to do A-B-A testing with any changes you make to test their effectiveness. Otherwise the results of your modifications could get lost or misinterpreted.
For example, you might add smooth wheel covers next week and go out and drive the exact same loop at the exact same speed. Then you go fill up and you got 33.67 MPG. Now you're bummed because your fancy new wheel cover seem to have made your mileage lower. But maybe it wasn't the wheel covers. Maybe it's 10 degrees cooler today and there is a 5 mph wind that wasn't there when you set your baseline.
With A-B-A testing you drive your test course without the wheel covers, with the wheel covers, then without the wheel covers again. Preferably back to back and with the exact same test conditions. It doesn't have to be for 90 miles either. If you're using your scangauge you could get accurate results driving a much shorter course.
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