Unexplained major drop in city fuel economy

I agree with your point, however, I don't see anything wrong with having the OP checked their 12v just in case. Even if it is not a sound theory, we could be preventing other debilitating problems in the near future by having the battery assessed. It will also help support or debunk the idea of the 12v battery having a large effect on FE.

 

But if you use the nav then the car is already on, first thing is pre-charge the break kind of big hit, and even in accessory mode the measurement will not be usefull. You need the open circuit voltage before anyting turned on.

Either it can be measured on the ODBC port (that is always HOT) or the battery itself. But you need to enter from driver side (that door doesn't trigger break pre-charge). Battery is little more work to gain access to but not hard. Or of course the jumping pont under the hood again you need to open it from driver side ... or if you have a secure garage pop it the night before.

 

If you bought your car new, you really don't know how many times the dealer might have let the battery run down and weaken the 12 volt battery. Most batterys only last 4-6 years. Prius 12 volt battery does not due much so a small drop in voltage is not very noticeable but may make the big battery keep trying to charge it, causing the engine to run more than normal, thus low MPG. My guess check the 12 volt battery.

 

web1b: +50 mpg @ 65 mph is about right.


1. Has your driving patterns changed?
a. Does your driving commute have any new road construction?
b. Does your driving commute have more traffic jams this year?
c. Are you driving any faster?
d. Are the road surfaces on your driving commute going bad?
e. Are you doing moe highway driving?
f. Are there more stop signs or traffic lights in the city route?

2. What is the make and model of your tires? (e.g. Yokohama Avid S33)
3. What is your tire pressure settings? (front and rear, in psi)

4. What is your driving location (general area)? More to the point... Are you driving in the North (e.g. USA) or South Hemisphere(e.g. New Zealand) ?

5. Has your Prius been in any car accidents?
6. Is there any signs of damage or collision damage on any of the tires?
7. Does your Prius make any noise when driving?
( Wheel alignment, damaged brake pads- wheels will lower fuel efficiency)

8. When is the last time you had your oil , oil filter, and air filter changed?
(mileage/date? good fuel efficiency depends regular maintenance )

 

This may be obvious, but do you have the "summer gas/winter gas" shifts where you live? We do almost 95% highway driving so I can't talk to city driving, but last fall during this switchover we noticed a larger drop than usual in our milage - same brand, same filling station. I'm guessing the refinery changed the gas formation, probably to save money. When we changed back this spring (about six months later), our milage returned to normal. If your milage has dropped for more than six months, this probably isn't the reason...

 

Web1b,

Did test 12v the battery? What was the result?

 

That's what I am waiting for. It's worth checking.

MPG Boost After 12V Battery Replacement? I Think So! | PriusChat

 

I am at a loss to understand what is happening in this thread.

FWIW, we don't even know the year/model of the OP'sPrius, nor the location of operation
-- service in areas experiencing long very hot or cold battery soaks, Arizona, Alaska, etc?

Members have suggested a number of items which need to be investigated if for no
other reason than to shorten the list of potential causes.
ATT, the list of items in need of investigation but as yet not acted upon includes
(post #s of member's suggestion/request):
1. Test the 12V battery, numerous requests
2. Dirty air filter, #8 -- will do this weekend; no results posted
3. 'Hard reboot, (disconnect 12V battery for a minute or so), #9
4. Overfilled oil, #16
5. Dragging parking brake, #16
6. Fouled/loose air intake duct work, #25
7. Dirty MAF sensor, #25
8. Dirty throttle body, #25
9. Dragging disc brake pads, #30

But the OP has taken no action, or at least posted no definitive yes/no answers, making
it impossible to narrow down the list potential causes, never mind actually identify the
problem. The OP shows an inexplicable reluctance or refusal to act in his own best
interest to get the help he is asking for. I just don't get it. ?????

Why is it so difficullt to get the OP to do the simple things needed to get to the answer
he seeks, if only by a process of elimination?

 

Web1b, my Prius is the same make and model as yours but I'm getting about the same fuel efficiency I got last year at this time. My Prius which has +36,000 miles on it is also still is running on its original OE tires - Yokohama Avid S33D ( 44 psi/42 psi) - but the tread is wearing thin these days and I'm thinking of getting new Energy Saver A/S this year. It's spring time here in the Washington DC Metro Area, so so my Prius' mileage is increasing - but because I'm hypermiling its a bit higher than most -- +60 mpg. Last winter was been brutal and my MPG was lower than in the previous winter which was much milder - so I'm pretty happy that spring has arrived. However, with Spring road construction induced traffic jams has also arrived and it is becoming more frequent here and it is really hurting my fuel efficiency. 10% ethanol regular gas is the standard here but some of the gas stations are now pushing 15% ethanol gas but I am avoiding E15 like the plague. We haven't yet switched over to the summer blend of gas yet as far as I can tell.

 

Maybe he is out of town.

We'll give him a little more time to get to it...