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Prius sudden gas mileage drop

My 2004 Prius quit (Danger warning lights) 2 miles after a fuel fill-up. The dealer said I’d run it out of gas and the 12 volt battery was no longer good. They replaced the battery and everything is fine except my summer mileage dropped from 47 mpg to 42 mpg. They said it’s getting old so that is not unusual.s A 10% drop from Friday to Tuesday??? That is pretty fast aging. I want my 47 mpg back. Any ideas??? Help appreciated.

If the 12v battery went dead, the ECM probably lost it’s memory and all the trim adjustments it stored. This means the ECM must readjust itself to optimize the fuel mileage again. This could take a few hundred miles. Check it again in a month.

PS - Wow, 5 different posts about declining MPG. Just in time for the return of summer blend fuels. The new formulations may also have a lot to do with it.

How many miles are on the car? When my mileage dropped like that I replaced all the cells in the HV battery and when I tested my old ones I found 8-10 cells going bad. It only cost around $1,400 and that is WITH buying the Autoenginuity Diagnistic PC software and hardware for Toyota. You can get new cells from wrecked low mileage cars from 04-09. Newer car battery cells may work but I have not heard of anyone using them.