Prius Mileage 2

[msquared48]

Just had my 60000 mile service about a month ago and my mileage has dropped significantly from around 52 to 42 now. I do not think that it's all the cold weather and gas additives, but I know that plays a large part.

Last year in similar weather I was getting 47 mpg. I think I'm going to take it in and see if the adjustments were properly made during that service. I'm suspicious.

Any other thoughts or recommendations?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ivymike]

hmmm... that's a big drop. how are you determining your mileage?

 

[msquared48]

By the averaqe mileage seen on the screen - computer generated. It roughly checks woth the mileage driven and the fuel consumed when I calculate it too.

I also reset the mileage computer at the last gas fill, so the computer average reading for the last 100 miles or so is very accurate.

It's disconcerting to say the least.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Peter7307]

msquared48 ,
There are a few Prius owners groups on the internet and perhaps one may hold the answer you are chasing.

This is the leading item according to Google:

http://www.priusownersgroup.com/

I have to agree that is a significant drop.
Wheel alignment alterations possibly?

Pete.

 

[RossABQ]

I assume Prius' have adaptive computer software; did they by any chance disconnect the battery? In other words, is the software re-learning?

 

[MacGyverS2000]

The computer should have re-learned anything it needs within a 100 miles or so... nothing it would be learning for any extended period of time would cause that sort of drop.

Make sure they didn't "upgrade" your ECUs firmware with any "fixes" from Toyota while you were in there. If they replaced the sparkplugs, did they gap them correctly? tire pressure within acceptable range? Quite a few things would have to add up to lose 20% of your gas mileage.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[ivymike]

It roughly checks woth the mileage driven and the fuel consumed when I calculate it too.

I've logged my fuel mileage in each of my recent vehicles over the entire period I've owned them. My experience has been that tank-to-tank variation in when the fillup stops contributes significantly to errors in a one-tank calculation (so use 3-tank averages), and that the computer estimate is only within 15% of the actual fuel economy when it gets lucky. The computers always seem to guess high - must be a marketing influence.

All that said, if the computer is saying mileage has changed, then something must have changed in its inputs, which don't include tank-to-tank shutoff variation. If they did upgrade the ECU, perhaps they have an adjusted algorithm for estimating fuel economy. Maybe you have an electrical circuit which stays on overnight, leaving you with a charge deficit in the morning? Would have to be a big one...

Are you still driving to work on the same route each day, or have you recently spent some time driving a different route?

 

[DanEE]

Try posting your question here..

http://priuschat.com/forums/

There's a specific forum dealing with fuel mileage and factors affecting.

Any changes in your driving route, frequency of driving coincident with the dealer visit?

While I suspect the following is not your root cause, it is something you probably would like to know about.

I've noticed (and one of the root mechanisms was documented by Oak Ridge National Labs engineering analysis of the Prius design) that short trips spaced out by greater than 1 day intervals can knock the mileage down noticeably e.g. from 52 mpg averages to about 46 over the same routes). The 1 day+ cycle is bounded by the characteristics of the coolant heat recovery system in Gen II design, 2010 Gen IIIs use a different mechanism there mainly to meet PZEV requirements.

While the 2nd gen Prius has a coolant heat recovery /insulated storage system (with a good +24hr cycle, infrequent trips result in initial start up energy loss to become noticeable i.e. back to back initial start up energy losses during the first 5 minutes of operation.. In addition to the energy just to heat the mass of the system, per ORNL studies, the oil windage losses in the spray mist cooling system inside the HSD drive goes up significantly until oil temperature comes up.

These losses would be characteristic in all vehicles but not as noticeable in less efficient vehicles.

 

[msquared48]

I am familiar with the Priuschat forum and have been a member for a couple or years. I wanted opinions other than the standard answers Toyota gives.

I am going to make a run of 100 miles or so today to the airport on the freeway. I'll see what happens, and if it stays the same, I'm going back to the service dealer as something is wrong.

Thanks for the information guys.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[thundair]

Have you changed where you buy your gas?
I have heard a few stories around work, with warnings about some stations.
I have never watched it that close but some of these guys at work are fanatic even about the time of the day that they buy gas.

Just a thought.

I don't know anything but the people that do.

 

[msquared48]

Just got back from the airport and now it's just over 48. That's acceptable, but I will continue to monitor.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[IRstuff]

It correlates with global warming...

TTFN

FAQ731-376

 

[GregLocock]

Firstly, over 100 miles the difference between 47 expected and 42 actual is about 1 pint of fuel. I'm not necessarily convinced that that is enough of a difference to trust. Does Prius use a returnless fuel system?

Secondly, won't you see a hit in mpg for whatever shuffling around was done at the garage?

Thridly did you top up at your usual staion. If not and there was a bit more ethanol than usual, that could be a few % straight off the bat.

Alternatively since you are coming into winter they may have switched to a more volatile brew, which may have a lower calorific value and higher octane.


Basically not enough data!

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[msquared48]

I always use the same local station for filling, exception is long trips of course, but that's not the case here.

I always top off too, generally adding an extra gallon or so after the automatic shutoff happens. Never been a problem. I also record the miles and amount put into the tank at each filling, and have since I bought the car in late Jan of 2007.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[IRstuff]

They should actually have been on the better blend since October or so. Southern California usually runs the poorer blend between May and November, if I recall correctly. Not sure what they do elsewhere; but we always wind up with price spikes when they switch over.

TTFN

FAQ731-376

 

[ivymike]

I also record the miles and amount put into the tank at each filling, and have since I bought the car in late Jan of 2007

Well then plot the single-tank figures, and add a 3-tank moving average line over the top, and look to see whether the recent "change" looks important in the grand scheme of things.

 

[msquared48]

I can do that, but why did you chose those figures. Statistically, why the three tank variation?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MacGyverS2000]

I always top off too, generally adding an extra gallon or so after the automatic shutoff happens.

Aren't you defeating the one of the purposes of a "green" car if you chance killing the evap system by overfilling?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[ivymike]

three tank moving average, not three tank "variation."

Three tanks smooths the result, but not so much that it obscures month-to-month trends. Two tanks gives a choppy line. A three-tank moving average is, of course, going to give a more accurate representation of average fuel economy over the period in question, but it takes longer to get (about 30 days for 3 tankfuls, in my case).

 

[DanEE]

"Aren't you defeating the one of the purposes of a "green" car if you chance killing the evap system by overfilling?"

There have been some cases of the fuel cell system being damaged by forced overfilling on Gen II Prius.

The Gen II does not have a conventional fuel evap system design but instead uses a fuel bladder inside a steel tank such that there is no gas fume saturated air to be expelled during filling as occurs with conventional car tanks with a gasoline/air/fume mix in the tank.

The bladder expands and contracts as fuel levels vary. An unusual visible aspect of this system is that when the fueling station gas nozzle is inserted into the filler, the nozzle makes an air tight seal with the fill pipe.

The Gen III (2010 models)uses a different system I understand without the bladder.