Gas mileage drop after new battery

Check that the voltage on the new battery is healthy, and if so it's probably unrelated to the battery itself.

There's almost an "epidemic" here of people reporting drastic MPG reductions after all manner of routine repairs and maintenance. The only common factor seems to be the disconnection of the 12V battery and the consequent (temporary) loss of ECU historical data.

As long as the new battery checks out healthy, then just wait a few weeks and your MPG will return to normal.

 

Three things you do not give details on in your reply. How did you check the voltage? voltmeter or MFD display? What state was the car in when the voltage was checked? Off, acc mode, ign mode, or ready? What was the voltage you registered.
Although you may have done the test correctly there is always the possibility you did not, and knowing these details also helps though's following you with less experience.

John (Britprius)

 

agree with britprius, if the battery turns out to be solid, you need to test a full tank and refill measurement.

 

Thanks for the update. For a new battery those voltages are rather low. A new AGM battery fully charged should be in the region of 13 volts off load, and about 12.5 or higher on load. A voltage of 14.3 with the car in ready mode suggests the charging system is working hard to charge the battery. This would indicate the new battery was low on charge when fitted giving you a low mpg figure. When charged it is normal for the Prius to have a ready voltage of 13.8 volts.

John (Britprius)

 

many complaints here of optima's coming with less than a full charge. you need to put a trickle charger on it overnight and retest.

 

I just replaced the 12v battery two days ago. Same issue with the display reporting low MPG (sub 20's) and I also got the red triangle light. A couple of hours after a long drive that same day, the light disappeared and everything returned back to normal.

 

rivets, the low MPG indication has been seen often enough in this situation, but not the red triangle. Have you checked your oil?