10 mpg loss --->SOLVED!

Yeah, my mileage has started to come back at a little less than that. Still a puzzle I guess.

- D

 

Old thread I know, but I just replaced 12v start battery and mileage jumped from 43 to 54 around town. Original battery lasted six years.

 

Thank you for posting this. It’s my daughters Prius and it was averaging high 20s to low 30s if I really babied the gas pedal. My son has 2013 that we bought an OEM battery. I swapped the batteries to check if this would solve the los MPGs. Good news it did. I took it out and within 10 minutes of driving the car hit 50.8 MPG. The battery had a 12/23 sticker and today it’s March 2025 which means it was only 15 months old. I guess the aftermarket is not as strong as the OEM and it pulls voltage from the hybrid battery. Replace your aftermarket with an OEM battery.

 

Old thread, I know, but seeing as the door has been opened ...

Unless you use a memory-saver device, when you disconnect a 12 V battery it resets a whole lot of stuff including the average MPG for the car. Just the act of replacing the battery means your average MPG is starting over fresh so you don't really know that the MPG is any better or worse than it was before the battery swap. You might have had the same miracle just resetting the average MPG. We will never know.

A 12 V battery will have only a limited set of fail conditions that will affect MPG. It might work in some cases, but it is not a silver bullet for all.