What accounts for this?

hobba · 06-20-2009, 08:30 AM

My last dozen or so tankfuls have yielded wild variations in gas mileage. Every other tank is very low mileage or high mileage. For example, 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. My previous tankful was 48.3 based once again in the Excel spreadsheet. This has been the case for each of the the last 13 fillups. One being high, the next being low mileage. I did have the dealer reset the inclinometer the last time I had service, but it hasn't affected anything. My travel is unchanged as is my routing and use of AC which is constant here in FLA. The first three years of ownership, my mileage varied little or not at all. Anyone else share this experience?

BDDave · 06-20-2009, 08:41 AM

If you use ethanol (89) vs non ethanol (87) you will see variations in MPG. The biggest change is I see in MPG is the wind; head winds, crosswinds, etc.

BRK · 06-20-2009, 11:36 AM

Don't forget with the bladder gas tank it is difficult to know exactly how much gas was consummed since it can trip off the gas pump prematurely and throw off your gallons of gas value.

Frayadjacent · 06-22-2009, 08:37 PM

Fuel formulations and tank bladder causing filling variations will be the biggest culprits... aside from the nut behind the steering wheel.

JimN · 06-22-2009, 10:15 PM

I suspect the pump & if you were in NJ, the attendant. Once it clicks off do you continue to top off? I don't top off & had very few wild swings.

qbee42 · 06-22-2009, 11:11 PM

Variation from the fuel bladder.

Tom

hobba · 06-23-2009, 09:00 AM

Thanks for the feedback.

I don't top off when fueling. Also, this is a relatively new phenomena. For 3 years I showed a consistant MPG and it has only been in the last dozen or so tankfuls that this has occurred. I question what has changed after that amount of time. I still avg. over 40 mpg which is fine with me.

qbee42 · 06-23-2009, 09:20 AM

The bladder stiffness could have changed with time and temperature, but more likely it is the gas pump. Tank venting issues can also influence fill levels.

Tom

richard schumacher · 06-23-2009, 10:17 AM

I'll bet a nickel that your MFD values have remained consistent, in which case it's variability in the size of a fill (bladder stiffness, wacky nozzle shutoff, a venting problem).

CBarr31 · 06-23-2009, 05:59 PM

Hobba,

As expected on a Prius forum you will here everythign about the gas tank bladder, pump differences, shutoff variations, time, temperature, tire air pressure, etc.. etc...

Don't get me wrong these are all very valid explanations BUT and this is HUGE. I have had the EXACT same problem you have had not only on my Prius but also on 2 different Corollas, a 4-Runner and my current Tacoma. Yeah I buy a lot of Yota's, LOL.

Anyway it isn't always one high then one low sometimes maybe most of the time they will be consistent from tank to tank but then they seem to get in a rut and just go one high then one low then high then low. It really is annoying.

BIG swing in mpg's on consecutive tanks - ???

That is the link to another thread that discusses it as well. My post in there says "I gave up." If you come across an answer please let me know.

Happy Driving,
Chris

06-20-2009, 08:30 AM	#1
hobba Member Join Date: Jun 2006 Location: Estero, FL Posts: 69 My Car: 2005 Prius Model: Package: #6 Thanks: 0 Thanked 1 Time in 1 Post Friends: 1	What accounts for this? My last dozen or so tankfuls have yielded wild variations in gas mileage. Every other tank is very low mileage or high mileage. For example, 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. My previous tankful was 48.3 based once again in the Excel spreadsheet. This has been the case for each of the the last 13 fillups. One being high, the next being low mileage. I did have the dealer reset the inclinometer the last time I had service, but it hasn't affected anything. My travel is unchanged as is my routing and use of AC which is constant here in FLA. The first three years of ownership, my mileage varied little or not at all. Anyone else share this experience?

06-20-2009, 08:41 AM	#2
BDDave Member Join Date: May 2008 Location: Iowa Posts: 70 My Car: 2008 Prius Model: Package: #2 Thanks: 0 Thanked 1 Time in 1 Post Friends: 1	Re: What accounts for this? If you use ethanol (89) vs non ethanol (87) you will see variations in MPG. The biggest change is I see in MPG is the wind; head winds, crosswinds, etc.

06-20-2009, 11:36 AM	#3
BRK Junior Member Join Date: Jan 2008 Location: Philadelphia (East Falls) Posts: 20 My Car: 2009 Prius Model: Package: #4 Thanks: 0 Thanked 1 Time in 1 Post Friends: 0	Re: What accounts for this? Don't forget with the bladder gas tank it is difficult to know exactly how much gas was consummed since it can trip off the gas pump prematurely and throw off your gallons of gas value.

06-22-2009, 08:37 PM	#4
Frayadjacent Resident Conservative Join Date: Mar 2009 Location: Austin, TX Posts: 327 My Car: 2009 Prius Model: Package: Base Thanks: 1 Thanked 14 Times in 14 Posts Friends: 3	Re: What accounts for this? Fuel formulations and tank bladder causing filling variations will be the biggest culprits... aside from the nut behind the steering wheel.

06-22-2009, 10:15 PM	#5
JimN P719 of ??? Join Date: Nov 2006 Location: South Jersey Posts: 2,691 My Car: 2006 Prius Model: N/A Package: #3 Thanks: 0 Thanked 28 Times in 26 Posts Friends: 2	Re: What accounts for this? I suspect the pump & if you were in NJ, the attendant. Once it clicks off do you continue to top off? I don't top off & had very few wild swings.

06-22-2009, 11:11 PM	#6
qbee42 Senior Member Join Date: Mar 2006 Location: Northern Michigan Posts: 10,468 My Car: 2006 Prius Model: Package: #7 Thanks: 93 Thanked 387 Times in 289 Posts Friends: 10	Re: What accounts for this? Variation from the fuel bladder. Tom

06-23-2009, 09:00 AM	#7
hobba Member Join Date: Jun 2006 Location: Estero, FL Posts: 69 My Car: 2005 Prius Model: Package: #6 Thanks: 0 Thanked 1 Time in 1 Post Friends: 1	Re: What accounts for this? Thanks for the feedback. I don't top off when fueling. Also, this is a relatively new phenomena. For 3 years I showed a consistant MPG and it has only been in the last dozen or so tankfuls that this has occurred. I question what has changed after that amount of time. I still avg. over 40 mpg which is fine with me.

06-23-2009, 09:20 AM	#8
qbee42 Senior Member Join Date: Mar 2006 Location: Northern Michigan Posts: 10,468 My Car: 2006 Prius Model: Package: #7 Thanks: 93 Thanked 387 Times in 289 Posts Friends: 10	Re: What accounts for this? The bladder stiffness could have changed with time and temperature, but more likely it is the gas pump. Tank venting issues can also influence fill levels. Tom

06-23-2009, 10:17 AM	#9
richard schumacher Destination: Eschaton Join Date: Mar 2004 Location: United States Posts: 5,507 My Car: 2004 Prius Model: N/A Package: #6 Thanks: 99 Thanked 93 Times in 66 Posts Friends: 0	Re: What accounts for this? I'll bet a nickel that your MFD values have remained consistent, in which case it's variability in the size of a fill (bladder stiffness, wacky nozzle shutoff, a venting problem).

06-23-2009, 05:59 PM	#10
CBarr31 Senior Member Join Date: Sep 2007 Location: Charlotte, NC Posts: 152 My Car: 2006 Prius Model: Package: #7 Thanks: 10 Thanked 4 Times in 4 Posts Friends: 0	Re: What accounts for this? Hobba, As expected on a Prius forum you will here everythign about the gas tank bladder, pump differences, shutoff variations, time, temperature, tire air pressure, etc.. etc... Don't get me wrong these are all very valid explanations BUT and this is HUGE. I have had the EXACT same problem you have had not only on my Prius but also on 2 different Corollas, a 4-Runner and my current Tacoma. Yeah I buy a lot of Yota's, LOL. Anyway it isn't always one high then one low sometimes maybe most of the time they will be consistent from tank to tank but then they seem to get in a rut and just go one high then one low then high then low. It really is annoying. BIG swing in mpg's on consecutive tanks - ??? That is the link to another thread that discusses it as well. My post in there says "I gave up." If you come across an answer please let me know. Happy Driving, Chris

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