Mileage has gone down dramatically

I am having the same issue with my 2006 Prius - mileage gone from 50 to 38 and all service done at dealer every 5K. I would appreciate any information or guidance to regain my mileage.

 

These threads are started with the usual questionaire:
Fuel economy complaints/queries? Please copy, paste & answer these questions, esp. if you're new | PriusChat

 

Not likely that the difference in oil is a factor at all........unless you put in something like 20W50 and it's still cold where you live.

If your 12 V battery is over 5 years old, I'd recommend changing it now.

After that, you need to get a scan done because a really LOT of things could be factors.......like a dragging brake just for instance.
Some will show up on a scan and some won't.
You need a good mechanic.......but those seem to be rapidly becoming extinct.

 

winter gas, bad weather, dirty throttle plate.