Causes of LOW MPG Average?

[kevinwhite]

Stephen,

Are you actually pressing the button on the MFD to reset the MPG display?

Although I have seen it written that it resets automatically after refueling, for me it seems to be that the distance reading resets to zero but the MPG figure does not unless I actually press the RESET soft button on the display.

If you didn't reset it then what you have displayed is the MPG figure since you bought the car. That figure will change very slowly over time.

After a reset the MPG figure changes very rapidly - go up a small hill and it will plummet, coast a bit and it will rise to a very high figure.

kevin

[savaged]

<div class='quotetop'>QUOTE(kevin17 @ Jun 26 2006, 11:06 PM) [snapback]277160[/snapback]</div>

Stephen,

Are you actually pressing the button on the MFD to reset the MPG display?

Although I have seen it written that it resets automatically after refueling, for me it seems to be that the distance reading resets to zero but the MPG figure does not unless I actually press the RESET soft button on the display.

If you didn't reset it then what you have displayed is the MPG figure since you bought the car. That figure will change very slowly over time.

After a reset the MPG figure changes very rapidly - go up a small hill and it will plummet, coast a bit and it will rise to a very high figure.

kevin
[/b]

Bingo. I'll bet that's your problem! (As mentioned, trip length is also a major factor I've found)

[Hutchings]

I have been only resetting the trip odometers, which in turn resetted the miles in the MFD as well. I have not reset it via the MFD, however that does make sense. We saw the miles driven reset and I guess we assumed that it reset the MPG figure as well. I will post back tomorrow when I explore that further. Thanks a bunch for this ( seemingly obvious ) procedure. I am anxious now to go try it except its quite late, and now past my bedtime.

[Marlin]

<div class='quotetop'>QUOTE(Hutchings @ Jun 26 2006, 07:40 PM) [snapback]277057[/snapback]</div>

95% of our trips are less then 15 minute outings.
[/b]

There's your answer. The shorter your trips are, the lower the MPG you will get. The longer your trips are, the higher the MPG you will get.

This is all because of the first 5 minutes of driving when the Prius runs less efficiently during its warm up cycle.

You have probably noticed on the Consumption screen that the first 5 minute bar on the graph is usually always between 25 and 35 MPG. Then the next bars are probably much higher, in the 45 to 60 MPG range. It's that first 5 minutes that is bringing you down.

Lets say you always make 15 minute trips and that you get 30 MPG for the first 5 minutes and 55 MPG for the last 10 minutes. If you do the math, you will find that (5x30 + 10x55) / 15 = 46 MPG. However, if your trips are always 20 minutes instead, you get (5x30 + 15x55)/20 = 48 MPG. 30 minute trips result in 50 MPG and 10 minute trips result in 42 MPG.

It's as simple as that. You can drive as carefully and smoothly as you want and you will still be limited by the percentage of your trips that is taken up by the warm up cycle.

[haceaton]

[quote=Marlin @ Jun 27 2006, 07:42 AM) [snapback]277287[/snapback]</div>

...
Lets say you always make 15 minute trips and that you get 30 MPG for the first 5 minutes and 55 MPG for the last 10 minutes. If you do the math, you will find that (5x30 + 10x55) / 15 = 46 MPG. However, if your trips are always 20 minutes instead, you get (5x30 + 15x55)/20 = 48 MPG. 30 minute trips result in 50 MPG and 10 minute trips result in 42 MPG.

It's as simple as that. You can drive as carefully and smoothly as you want and you will still be limited by the percentage of your trips that is taken up by the warm up cycle.
[/b]

While the jist is correct, the actual math doesn't usually work that way (unless you have a constant speed through each 5 minute interval). This is why I wish the MFD display showed 5 mile intervals instead of 5 min. Many 15 min trips might have a speed around 30 MPH for the first 5 minutes (getting to the highway), then 60 MPH for the next 5 minutes (on the highway) then 30 MPH for the last 5 minutes.

30 MPH x 5 min = 2.5 miles (0.083 gals of gas] 30 MPG)
60 MPH x 5 min = 5 miles (0.0909 gals of gas @ 55 MPG)
30 MPH x 5 min = 2.5 miles (0.0454 gals of gas @ 55 MPG)

Total distance = 10 miles, total gas usage = 0.2197
trip economy = 45.5.

What this shows is that the first 5 minutes usually hurts even worse than you think because it's usually done at low speed in neighborhood streets where you burn gas to heat the engine without travelling very far. The generation 2 prius has a thermal storage tank for saving hot water to reduce this problem, but it only helps a little.

[Hutchings]

Tuh-Duh! We reset the mileage on the MFD before we left this afternoon, and now I am getting the results I was looking for. I did not realize that you could reset the "lifetime" mileage. I guess it was a lifetime reading since I had never reset it. Now, for our 15 minute trip to grandmas house, she averaged about 52-54 mpg. I now reason that had we not reset the MPG reading, that it would have eventually come up as our mileage consistantly improved. I now see the procedure for an accurate reading from tank to tank, which shows us the results of our driving patterns. My wife is pleased again, and I feel much better that we are actually getting the better mileage you all have boasted. I am walking a bit taller this afternoon and feeling quite proud again! Thanks for the offerings and sugestions you haved all posted. I read aloud with my wife, the article on factors that imped you mpg and will start on some of those ideas too. I like that comment that brakes are only for emergencies! Thanks again all, hope to run into you all again soon on the boards.

Stephen

[rak]

In our first 6 months (2005 model), we never got better than 35 mpg (yikes!), and now, it's tough to even pass 44 - so you're already ahead of me. After reading other posts, I realize it's mostly because my trips are very short, and we have stop signs or pedestrians crossing or unavoidable stop lights at least 4 times per mile. I've been trying to do more of the glide thing, but it doesn't seem to be enough to make up for the things I mentioned. On the other hand, others around here (Berkeley) do get in the 50's - maybe their trips are longer?

[Maytrix]

Originally Posted by Hutchings

Tuh-Duh! We reset the mileage on the MFD before we left this afternoon, and now I am getting the results I was looking for. I did not realize that you could reset the "lifetime" mileage. I guess it was a lifetime reading since I had never reset it. Now, for our 15 minute trip to grandmas house, she averaged about 52-54 mpg. I now reason that had we not reset the MPG reading, that it would have eventually come up as our mileage consistantly improved.

This must be new as my 2005 resets both the MFD mileage and average mpg readout each time I fill up.

[rumpledoll]

We are the same way. My wife uses the car during the week, and the vast majority of her trips are shorties with the car barely going beyond that first 5 minute bar. Are mileage here in NJ (2007 std. Prius) is ~34MPG winter and ~43MPG summer. When we take out the car for longer trips and P&G, the mileage can often be 50+.

On the other hand, we still keep our 2002 Ford Explorer for those hauling needs (e.g. taking my telescopes and gear out to a dark sky location, Prius cannot hold that stuff) and I used it this past week doing the short trip driving and it now has 9MPG for the ~60 miles - so we are kicking butt with the Prius!

Originally Posted by rak

... I realize it's mostly because my trips are very short, and we have stop signs or pedestrians crossing or unavoidable stop lights at least 4 times per mile.