2006 Prius huge MPG drop. Why??

who drives the car more, you, or your bride?

 

I highly doubt it got 50-55 miles per US gallon. I don't know what units you're using.

If it was 50 miles per Imperial gallon, that == ~41.6 miles per US gallon. That's 34% above the US EPA highway estimate for that car. That's might be possible on very long highway trips at low highway speeds (say 50-55 mph). If you claim to get 50+ miles per US gallon on that car, either you've got some big sources of error (e.g. odometer way off), you're calculating incorrectly or you're very good hypermiler.

Yes, using B mode generally hurts FE.

As for 12 volt, we've found that dealer "tests" of the 12 volt are useless. If you have an 06 Prius and are on the original 12 volt, it's most likely gone bad by now. I changed mine on my 06 a bit over a year ago (http://priuschat.com/forums/gen-ii-...id-seattle-area-installers-2.html#post1264180). It was already getting marginal a year ago.

 

You may need to watch which Prius they have. As a general rule, the Gen 3 does 5 MPG better than our Gen 2. You may also wish to notice where they live, it was 71F (22C) here today, that is not 'winter' in Ontario.

The 12 volt battery runs accessories when off (never do this, stay in READY) and starts the computers so you can get to READY when you press Power. As a general rule they may last about 4 years. SOME of the failure modes for a bad 12 v Battery cause low MPG, as the car attempts to charge it continuously. It is about $130 to $170 USD.
http://www.elearnaid.com/opdsopbadifi.html

BTW, there is a cheap stiffening plate to help wander in winds.
prius g7 | eBay

 

that's why i asked, my wife is the same way.

 

my 2008 Prius consistantly got 47-49 mpg with heavy footed wife driving. Now after 42K miles it has suddenly dropped to 37-39mpg, same wife - same foot, any ideas? Winter not really a factor since we are having a milder than usual year.

 

No thats not going to tell you much. You must measure under accessory load which is step #2. Second button push.
But 12.3 without load is not good at all anyway. I would expect it to be 11.7 under load. Way past replace.

Also I can tell you I have had excellent success restoring mileage by replacing the Inverter Coolant which is really the heart of the car.
You never mention how many miles on the car but that coolant is only good for about 50K miles IMHO.

That and you probably have a very dirty throttle body too which can create classic under slight load stumble.

 

Somewhere on this site is a set of instructions on how to test the 12V battery a better way, using the MFD screen (e.g. no multi-meter needed). I used the instructions a few months ago and confirmed my original 12V dated 2003 on our 2004 Prius was indeed bad. At no load, it was showing around 12V but under load of just the headlights (HID), it dropped to 8.9 instantly. If you haven't already found these instructions, please try them. They are a quick and non-evasive way to check operation of the 12V battery and will show you how to get into the top secret maintenance mode.

 

I'm guessing your 12v battery is on its way out. The engine has to run more often to keep it charged. I was experiencing a nearly 10 mpg drop, and replaced mine (easy to do), and bada-bing - back up to 50s for mpg.

 

12V battery

12 v battery check combination