What accounts for this?

For those cases you mention can you give the specific calculated and MFD values? Are they roughly 51 MFD, 51 calc., and 26, 26? That would indeed be pretty alarming unless you had some sort of assignable cause like dragging a battleship anchor for the whole tank.

Or are the pairs more like 51, 26 and 26, 51 respectively? This would indicate tank fill/gauge variability.

 

for one thing, the bladder will always provide inconsistent fills, so only look at the average over a minimum of 3-4 tanks.

also, the next time you fill up and get about the same mileage on the pump AND the MFD. remember that for your next fillup because that is mostly a good fill.

now, suppose you drive 450 miles and MFD says 50 mpg... you fill up, it stops at 8 gallons, what do you do? stop? well, what i do, is i say, i should have used 9 gallons, so i will go up to at least 8.8 gallons or so. if you dont, you will most likely be short and the next tank will fill more making it look like you have mileage in the mid 30's...to be honest with ya, that is not an easy thing to do. only during the extremes of winter do i even get close to that... (actually only got as low as 39 mpg). for you, its the hot weather and right after i got my 2004 Pri, i was new to hybrids so wasn't used to getting great mileage. had a hot spell of 3 days (twice at or over 100º) then went to Montana where it was in the 90's for another 3 days. had the A/C blasting and still got almost 50 mpg

the reason i started doing this is because some pumps dont work correctly and auto shut off will happen too early. after it clicks off once, your choice is to trickle in the amount you feel should be in there, or wait about 5 minutes for any fluid in the valve to drain back so it will work properly again.

 

So are you saying that your *actual* mileage is varying between 50s and 20s? Let's get this straight so that we know the real question.

Tom

 

Yes, that is what I was asking as well. The two consecutive tanks mentioned previously were very curious and could provide a basis for narrowing the focus.

 

1. So what happened last May, just before this started?
2. Unless Toyota USA has already sent an engineer to diagose it, take it to another dealer.
3. Has anyone looked at the throttle plate and the mass air flow sensor?

 

Hobba,

When rational answers can't be found, it's time for a conspiracy
theory.

Are you the only driver of this car?

Is it possible that at random intervals/distances another driver cranks
the air co way up?

Could that same person be also adding some fuel -- a couple of gallons,
$5 =~2 gal -- at random intervals?

For the latter, I'm going on the idea that when empty the car needs
more than 3 gals for it to to be shown on the guess gauge. In turn, if
it's not registered on the guage, then maybe it wouldn't reset the miles
travelled on a tank display, and would then go unnoticed by you.

A long shot for sure.

 

Hobba, that post could really use some spaces, commas or something to put it in a recognizable form.

 

Hobba,

Those are similar swings which I occasionally see in Ema but regularly saw in my Corollas and 4-Runner, as I noted before and in previous posts. Your last 4 tanks seem "consistent" you might be out of the high/low rut as I call it.

Like I said before "Good Luck" finding an answer. I've seen this anomaly for more than a decade in 5 different vehicles and have talked about it with a dozen or more highly knowledge people and all we have got out of it is guesswork.

Enjoy driving and don't think about it to much or it will drive you nuts, LOL.

Chris

 

Something changed to cause this. Eventually it will manifest in some more easily diagnosable fashion. Please let us know of developments.

 

looks like you are averaging 42 mpg. your wild fluctuations can probably be attributed to filling errors which are caused by faulty gas pumps (a study showed that the auto fill valve on 20% were defective) not filling to a consistent level.

now is this something new that is unique to the Pri? nope...its been going on as long as cars have been around. we used to contribute noticeable short tanks to "bad gas" etc... well, that was never the case because true bad gas would have most likely ruined the engine. we simply have more info and feedback on our driving performance.

based on your locale, you run AC, may be in a densely populated area, etc. so your mileage is not really all that bad. my dad averages the same as you do. he lives in Mission Texas which is a stones throw (if you are a girl) from Mexico. his mileage is bad, partially because he has degraded vision which causes him to drive without much anticipation....but then again, he had a 2005 Camry that barely made 20 mpg around town, so he is still doing much much better (his Pri is only a 2007)

a true comparison requires you to reset each tank as you fill it and record what the MPG's are as well

 

actually there are several "possible" answers and the true answer is probably a combination of several things. you are doing all you can do and that is collecting data.

ultimately, its how much you take out of your wallet to keep from having to walk.

 

Is it possible to steal gas out of a Prius. LOL

 

Check the oil level. When you got your oil change, they may have overfilled it.

 

Earlier hobba stated: "The MPG as calculated by the MFD vs. the spreadsheet have varied by 0.2 to 0.4 MPG." Something is not right with this story as that is at least an order of magnitude less than the typical fill variation I've observed. I've had exactly one tank that showed only 0.4 mpg delta between the fill and the MFD. The median variation has been 2.3 mpg. And the standard deviation (if I throw out the one incomplete fill--car refused to fill) comes in at 2.4 mpg.

Also, the above doesn't mesh with the original post: "My most recent tankful claims 35.5 mpg based upon the Prius excel spreadsheet available on this web site. The MFD says I got 41.6 MPG, but I only drove 363 miles."