I avg 38mpg on my 2005 prius.

How do you know the battery is fine? What are the voltage readings after sitting up for some hours, like overnight?

 

Get the voltage values with this on-board method. Weird stuff happening? MPGs dropping? Test The Battery | PriusChat and report all three values back here so others can help you determine if the 12v is weak.

In the Prius, with any strange electrical or mpg issue always suspect the 12v until that possibility is eliminated.

 

The 14V is only telling you that the charging system is working.

As SteveLee posted above, please report all three values, not just the last one.

DO-IT-YOURSELF TEST PROCEDURE (thanks to jdenenberg):
- Without brake pedal, press the Power button once and release to enter ACC mode
- Press and hold the MFD Info button, then turn the headlights on and off three times to enter Maintenance mode; release Info button
- Press “Menu” (on screen)
- Press “Display Check”
- Press “Vehicle Signal Check” - the battery voltage is shown and should be about 12.4 to 12.8 Volts (normal for an unloaded battery)
- Again without brake pedal, press Power button and release to put a current load on the battery - the voltage should stay above 12.0V (if less than 12.0V the battery is not well, or there is a fault or unusual load somewhere)
- Press brake pedal and press Power button once to enter "Ready" mode - the battery is now charging at about 14V (if less than 13.6V or more than 14.4 there may be a problem with the charging circuit)
- Turn car OFF to leave Maintenance mode
What we are wanting to know is the ACC mode reading, the ign on reading, and finally the ready mode reading. Follow the steps above and report back with all three readings.

Example: Acc Mode: 12.6 volts
Ign. On Mode: 12.4 volts
Ready Mode: 14.0 volts

Ron

 

Get the voltage values with this on-board method and report all three values back here so others can help you determine if the 12v is weak. I believe your 14 volts is the charging voltage. It's good that it's charging but that is not telling you everything you need to know.

 

Saithus,

It might be a good idea for you to read the following link. Fuel economy complaints/queries? Please copy, paste & answer these questions, esp. if you're new | PriusChat Then copy and paste the questionnaire into the post you started here. With the info from the questionnaire, we may be able to pinpoint if you have a problem or not.

YMMV

Ron

 

For a working vehicle in near constant service what you are asking for here is almost an impossibility.

All modern vehicles are complicated machines, The Prius probably more so than most. Therefore for all of us, there is always the constant risk of it quitting on you one day.

Significant investment into maintenance reduces that risk, but for an aging vehicle being used as a daily driver there is no way to totally divorce oneself from the risk of an unforeseen failure.

The catch 22, is nobody wants a high repair bill, but if you are saving money, or on a budget than often maintenance costs can be prohibitive as well.

That's one thing I enjoyed about owning some of my older small pick-ups. They were primitive in both safety features and amenities by todays vehicles standards, BUT...they were relatively simple machines. What could go wrong with them was very obvious. Engine, Transmission and maybe suspension...but there wasn't a whole lot else to worry about. Keep up with brake pad and clutch...and routine fluid replacement and they were bullets of reliability.

 

I suggest a spark plug change. Brought our GenII's mileage back up to mid 40's.

 

Have you driven it today? These numbers are a little weak but not terrible. Has it been charged today? I would consider filling out the questionaire now like dorunron suggested. The more info we have the better we can help.

 

As SteveLee suggested, please fill out the questionnaire. It will help immensely in your case. FWIW, your 12 volt is reading weak. What that means is that it is not FULLY charged. The 12 volt may last through the summer and fail during the winter, or it may last a couple more years. However, until you fill out the questionnaire we can only guess as to what the problem or problems is.

Ron

 

Yeah, actually 12v represents about a 50% state of charge for this battery. If it will not hold a charge it needs to be replaced when you can. A trickle charge may be an option but I think the general experience is that when they start going south they don't revive. At this point it could start causing weird electrical issues and loss of mpg (oops, already have that) and even put you down at the least convenient time. Better to replace it on your schedule instead of being forced into it. Gives you time to do some research for a replacement.

 

Synthetic oil of the same grade will have negligible effect on your fuel economy. That said however, it's still worthwhile using a good quality oil.

BTW. What is your oil change interval? (miles/months).

 

Soooo....whens the last time you checked/adjusted your tire pressure?

 

Your MPG expectations are reasonable. The 12V is weak and requires the engine to run more. Front end pulling, out of alignment causing more drag, makes the system work harder. Integrity tires have the best dry road stopping and cornering numbers but the worst on wet roads of a half dozen brands and is a couple of MPG's less than the Michelin ES and the Bridgestone Ecopias.

You're losing MPG by not taking advantage of the regenerative braking. The HV battery has to be continually recharged. The less regen braking is allowed to work, the more the engine has to run to charge it. Anticipating stops and slowing conditions far ahead by letting off the accelerator, coasting more, and then slight brake pressure allows the regen braking to recapture energy already purchased, saves on brake repair/replacement, and increases MPG.

It does take a little adjustment in driving patterns to take advantage of the efficiency of the car. It looks like the culprits are partially mechanical and partially driving style.