Logged my first tank in 2007 Prius

<div class='quotetop'>QUOTE(awakened @ Sep 13 2007, 01:34 PM) [snapback]512006[/snapback]</div>

Well, generally believe your math. Some people say the MFD is always high, but for me it seems to average about the same. Sometimes higher, sometimes lower. I think the differences can at least partly be accounted for by two things:
1) this fill-up wasn't exactly as full as the last fill-up. Obviously if you underfill compared to last time, your mileage will go up, and vice versa. That's why using a 3-tank average is considered a truer reading.
2) the bladder (flexible gas tank) for American (North American?) market changes size depending on the weather, so if you fill up once and then a couple weeks later fill up when it's 20 or 30'F cooler, you won't be able to put as much gas in as the first time.

 

<div class='quotetop'>QUOTE(awakened @ Sep 13 2007, 01:34 PM) [snapback]512006[/snapback]</div>

Well, generally believe your math. Some people say the MFD is always high, but for me it seems to average about the same. Sometimes higher, sometimes lower. I think the differences can at least partly be accounted for by two things:
1) this fill-up wasn't exactly as full as the last fill-up. Obviously if you underfill compared to last time, your mileage will go up, and vice versa. That's why using a 3-tank average is considered a truer reading.
2) the bladder (flexible gas tank) for American (North American?) market changes size depending on the weather, so if you fill up once and then a couple weeks later fill up when it's 20 or 30'F cooler, you won't be able to put as much gas in as the first time.

 

<div class='quotetop'>QUOTE(awakened @ Sep 13 2007, 01:34 PM) [snapback]512006[/snapback]</div>

Welcome to Prius Chat!

For further discussion on your question follow this link:

http://priuschat.com/index.php?showtopic=38141&hl=

In general you'll find that for any individual tank of gas, the MFD will be more accurate. For long term averages across multiple tanks the calculation of total miles traveled divided by total gallons pumped will give you a good calculation.

One of the problems with trying to calculate MPG for an individual tank from the amount you just pumped in is that it really isn't a very accurate estimate of how much fuel you have used.

There are variations due to the bladder in the Prius gas tank expanding and contracting from various environmental factors as well as the fact that the pump handle might click off a little sooner or a little later from one fill-up to another.

If you are really curious track both sets of numbers over a long period of time and see how close the long term average gets eventually. Those around here who have done that generally indicate that the MFD differes from their calculations by less than 5%.

 

<div class='quotetop'>QUOTE(awakened @ Sep 13 2007, 01:34 PM) [snapback]512006[/snapback]</div>

Welcome to Prius Chat!

For further discussion on your question follow this link:

http://priuschat.com/index.php?showtopic=38141&hl=

In general you'll find that for any individual tank of gas, the MFD will be more accurate. For long term averages across multiple tanks the calculation of total miles traveled divided by total gallons pumped will give you a good calculation.

One of the problems with trying to calculate MPG for an individual tank from the amount you just pumped in is that it really isn't a very accurate estimate of how much fuel you have used.

There are variations due to the bladder in the Prius gas tank expanding and contracting from various environmental factors as well as the fact that the pump handle might click off a little sooner or a little later from one fill-up to another.

If you are really curious track both sets of numbers over a long period of time and see how close the long term average gets eventually. Those around here who have done that generally indicate that the MFD differes from their calculations by less than 5%.

 

<div class='quotetop'>QUOTE(nerfer @ Sep 13 2007, 01:51 PM) [snapback]512023[/snapback]</div>

:blink:
So you start by saying that the calculation of miles traveled divided by gallons pumped is the one that should generally be believed and then follow that up by stating 2 reasons why that method is likely to be less accurate?

I agree that a running average across 3 fill-ups would probably reduce the margin of error to something reasonable assuming that you are referring to fill-ups in the area of 9.5 gallons and not about 5.8 gallon fill-ups.

With 3 fill-ups of 5.8 gallons each and the uncertainty of fuel used somewhere around a half gallon you are looking at hand calculations that give you anywhere from 46.6 mpg to 49.4 mpg for an actual fuel efficiency of 48 mpg

With 3 fill-ups of 9.5 gallons each and the same uncertainty of about a half gallon, the hand calculations come out between 47.1 and 48.9 for that same actual efficiency of 48 mpg.

 

<div class='quotetop'>QUOTE(nerfer @ Sep 13 2007, 01:51 PM) [snapback]512023[/snapback]</div>

:blink:
So you start by saying that the calculation of miles traveled divided by gallons pumped is the one that should generally be believed and then follow that up by stating 2 reasons why that method is likely to be less accurate?

I agree that a running average across 3 fill-ups would probably reduce the margin of error to something reasonable assuming that you are referring to fill-ups in the area of 9.5 gallons and not about 5.8 gallon fill-ups.

With 3 fill-ups of 5.8 gallons each and the uncertainty of fuel used somewhere around a half gallon you are looking at hand calculations that give you anywhere from 46.6 mpg to 49.4 mpg for an actual fuel efficiency of 48 mpg

With 3 fill-ups of 9.5 gallons each and the same uncertainty of about a half gallon, the hand calculations come out between 47.1 and 48.9 for that same actual efficiency of 48 mpg.

 

The oil-change recommendation is 5000 miles for the Prius because unlike regular cars, the gasoline engine in the Prius is not running all the time.

I love how low-maintenance the Prius is. :wub:

 

The oil-change recommendation is 5000 miles for the Prius because unlike regular cars, the gasoline engine in the Prius is not running all the time.

I love how low-maintenance the Prius is. :wub:

 

Hi Awakened,

The gas tank bladder will stretch some with usage when new. So, for the first 10 tanks or so the MFD will always read better mileage. After a while in constant temperature conditions, the two will be similar. If there is any temperature change, however, the pumped mileage may read high or low from the MFD mileage. For example, my last tank was 58.6 mpg, but the pumped mileage was 62.3 mpg. That is because at fillup time (about 5 pm) the temperature this time was 65, versus 90 F when I pumped last.

During the middle of the tank as fall temperatures were slowly occuring here, it was like I was using no gas at all. As the bladder shrank, the gauge stayed in one place, right at 5 bars for 3 days. The middle reading of the gas gauge is where it usually changes relatively quick. 9 to 8 bars is the quickest, it seems.

 

OK - here's my first tank experience - which was interesting and leads to a couple of questions about the consumption screen on the MFD

I pumped 6.7x gallons. I was at 2 pips on the gas guage. According to the consumption screen on the MFD, I had gotten 48.x MPG. When I did the calculation manually, it came up to 54.x. A big difference - I'm thinking that with 2 pips, I should have pumped more than 6 gallons.

I realize that some of this is probably the fuel bladder and it's position in the tank at the time of fillup.

The other thing that I observed was that after I filled up, it reset the mileage on the consumption screen ( which after looking in the owner's manual, it's supposed to do). What the owner's manual doesn't say is whether the MPG on the consumption screen also resets.

So, here are my questions:
1. Even if I track my manual and the MFD numbers, which are to be believed?
2. Does the MPG on the MFD also reset when re-filling like the miles?

Thanks!!!!

 

<div class='quotetop'>QUOTE(07seaside @ Sep 16 2007, 12:49 PM) [snapback]513353[/snapback]</div>

1. For any individual tank, the MFD is likely to be more accurate. Over the long term, distance driven divided by gallons pumped will be a relatively accurate "lifetime" average. Most of the people around here who track both have reported that after many fill-ups the two methods differ by less than 5%

2. I believe this changes by model year. Those with 2005 have reported that the MPG does automattically reset. Those of us with 2007 have found that it doesn't.

 

Danny,

Thanks for the answers.

If your answer to question 2 is correct, and the MPG doesn't re-set when you fill-up, then you'd always have to figure it manually - if it doesn't re-set, it would be a cumulative number, not for that tank.

Is the solution to hit the re-set button on the MFD everytime you fill up?

 

In relation to oil changes, the Australian Prius spec for oil changes is 10,000km or about 6,000 miles. I think the regular change of oil is the most important thing, the 5000 mile interval isn't all that critical. The cars we use at work have a 15,000 km service interval. With modern close tolerance engines and the excelent oils availabe today servicing more often than 5,000 mile would be wasteful

 

<div class='quotetop'>QUOTE(07seaside @ Sep 17 2007, 08:37 AM) [snapback]513641[/snapback]</div>

Yep,

If you want a cumulative number, don't hit the reset button. If you want individual tank numbers, hit the reset every time you fill up.

If you are using the display for a cumulative number, keep in mind that if you ever bump the reset button accidentally you will lose that history and begin a new cumulative MPG from the moment you reset it.

For this reason, I keep a simple spreadsheet. The spreadsheet allows me to see individual tank MPG, cumulative MPG over any time period, total fuel used over any time period, as well as miles per dollar and cents per mile.