poor fuel economy with random

Hello, my car used to get 35 mpg summer, 32 winter, now I'm getting 28 and 25. No engine codes, seems to be running fine. I have a scangauge and the weird part is that when I coast down long hills in neutral, my mpg will jump around 120, then 180, then 90, 130, 220, 180, always changing. although everything else is the same, RPM, SPEED, TPS. Any ideas what could be the cause of an inaccurate reading?

Thanks!