MPG Decline/Bleed Overnight?

Hi all, first time poster. I recently bought a 2005 Prius Gen 2, and love the car so far.

The car drives beautifully and seems to be in fantastic condition. However, there is one thing that troubles me.

If I park the car for the night, and on the "Consumption" page it reads, say, 49.0 mpg when I shut it off, the next morning when I start the car again, the MPG will have declined somewhat, to say 48.2 MPG. This seems to happen every time I park the car overnight. If I park it for two days, the MPG count will have dropped even further, to say, 47.6 MPG. Note that these lower MPG numbers appear immediately upon starting the car, without using any gas.

What is going on? Is this normal behavior? I live in SoCal, and it's been 85-90 degrees every day, so it's not like the car is struggling with winter temperatures.

 

Thanks xpcman and cwerdna, I will do that test. However I have no trouble starting the car; has a depleted 12v battery caused the MPG counter to drop overnight, with the car being completely off and left alone?

Note: I've gotten 50 and even up to 60 mpg from this car on 25-30 mile trips, so I don't think there's anything wrong with the efficiency of the car. It's just when the car is left alone overnight, I get this "mileage bleed."

 

Ok, good to know. But, has this ever been known to cause the MPG computer to revise down its previous tally? Does anyone have an explanation for that behavior?

In other words, I understand the logic behind a bad 12v battery having parasitic loss overnight, and the voltage thus being lower, which might cause my subsequent driving session to truly give me worse MPG, because the battery is weaker and thus the hybrid system is not operating well.

But this seems a little different. The moment I press the "Power On" switch, the MPG I see is instantly lower than it was the night before.

So maybe the question becomes, is the state of the 12v battery, or the hybrid battery, somehow factored into the MPG the Prius is reporting?

 

Thanks Ken, I will look into the Optima if it seems my battery is weak. And I'm keeping a manual log with my gasoline consumption/mileage as you suggest.

I'm still hoping to hear from other forum members about this, to positively identify the problem.

And I have a hard time believing mine is the only Prius that does this. When everyone else parks their Prius for the night, does your MPG readout remain exactly the same the next morning when you power on your Prius?

 

Ok. So I ran the test for the 12v battery using the Maintenance Mode instructions from above. Here are the results:

1. Pressed "Vehicle Signal Check" in ACC mode: 12.2v (low; should be 12.6v)
2. With AirCon + Headlights + Rear Defrost On: 10.7v (low; should be 11.3v)
3. In IGN-OFF mode (two presses of "Power" with no brake), nothing extra turned on: 11.9v (low; should be above 12v)
4. In "Ready" (Engine Running), nothing extra turned on: 14.0v (good; should be 14v)

 

Thanks Uart. Actually I already did that test in #3 above but called it IG-OFF by mistake. I was running at 11.9v.

So now the question becomes, does using an Optima battery void the warranty on the Prius Hybrid components? I still have a lot of years and miles left on my warranty for that.

 

Another thought - could the MPG bleed I'm seeing have something to do with the fuel bladder having a leak of some kind? There isn't fuel leaking from my car, visibly; so it would have to be evaporating.

This seems highly unlikely to me, but would explain why the MPG number would drop overnight while parked; literally there is less fuel in the tank, so the computer would recalculate its MPG tally based on that lower amount of fuel.

 

Thanks again uArt. I checked the paperwork of the car, and as far as I can tell, the last time the aux battery was replaced was in Jan 2009. So that's over 3.5 years. Which is consistent with my needing a new battery, right?

Also to answer your question - the number of miles it's calculating the MPG for is indeed low. However I observed the same behavior when there were more miles on the previous tank. But the dip was less, because the denominator was of course larger.

I don't think the behavior is explained by what I'll call your "late data" theory, because it seems the MPG tally will dip more if the car is left for longer. I left it for 2-3 days without driving once, and the MPG had dropped 2x-3x more than it would have if it'd been left for only one day.

Also - can anyone answer my question about whether installing an Optima battery somehow voids the Hybrid Parts warranty? And is an Optima battery actually markedly better than the Toyota battery? This is a critical bit of info.

 

I was just informed by a dealer that it might void the warranty on my hybrid battery/parts, if I were to install a non-Toyota 12v battery. So that might answer that. Can anyone confirm this?

I was also quoted a price of $340 parts and labor for the new 12v battery by this dealer. Is it me, or is this a total rip-off?