My Prius reporting wrong MPG

ddockery · 10-13-2005, 05:38 PM

I've had my Prius since early Feb and am at just over 10k miles and just had my 2nd oil change. Lately, I've been doing my own milage calculations. Over 100 miles, it reported the MPG at ~40 when it was actually 50. I tried it again on a 300 mile trip and it reported 49MPG when I really got 39. Is this type of fluctuation normal? I don't expect it to be perfect, but this seems to be off by more than should be typical.

Thanks,
Drew

orangeblood · 10-13-2005, 05:58 PM

The big variable is how much gas you put into the tank. You may have read about the bladder the Prius has for a "tank," and the varying amounts of gas you can get into it each time.

If it happens to click off early when there is still room for another couple of gallons in the tank, your mileage will look better than it should. Likewise, if you are able to squeeze more than normal into the tank your calculated mileage will be reduced.

Over time these variations should even out, but I trust the computer-generated MPG figure on a tank-by-tank basis.

OB

9G-man · 10-13-2005, 05:58 PM

Could you clarify how you are determining actual MPG at the 100 and 300 mile intervals?

DanMan32 · 10-14-2005, 02:34 PM

Lets assume for a moment that the MFD is accurate.

For your 100 mile trip, the MFD reports that you used 2.5 gallons (100/40) where you reported using 2 gallons.

For your 300 mile trip, your MFD reports 6.12 gallons used while you report 7.69 gallons.

But lets add them up.
MFD says you used 8.62 gallons while you filled a total of 9.6 gallons.
It is quite possible you could have a difference in fill by as much as a gallon.

mikepaul · 10-14-2005, 03:35 PM

This goes against all things Green, but...

I cram the tank full of gas in order to be sure it's full. I stop when I see gas left near the top. I then drive until my MFD display shows I've driven 10 times the current MPG average, then fill up again, hopefully using 10 gallons.

I've never varied more than a half-gallon when I do this, and it's always been more gas than I expected to pump, not less. Given bladder stiffness and other variables, I don't think I'm ever off more than 5%. As I've got no reason to keep a journal of MPGs, I only check this occasionally.

When it's almost dead-on accurate, which is often, I'm happy...

ddockery · 10-17-2005, 09:52 AM

I didn't realize that filling the tank could very by that much. That's probably making the difference. Thanks for the heads up.

john1701a · 10-17-2005, 10:01 AM

Quote:

Originally posted by mikepaul@Oct 14 2005, 02:35 PM
I cram the tank full of gas in order to be sure it's full. I stop when I see gas left near the top.
[snapback]141155[/snapback]

Please tell us when your emissions sensor fails, because it eventually will.

It was designed with the intention of sensing vapor, not to be submersed in liquid.

My parent's purchased a rental Corolla earlier this year. Their sensor failed at just 31,500 miles. Overfilling the gas tank (which floods the emissions canister and all the piping) was concluded to be the most likely cause.

mikepaul · 10-17-2005, 11:00 AM

Quote:

Originally posted by john1701a@Oct 17 2005, 10:01 AM
Please tell us when your emissions sensor fails, because it eventually will.

[snapback]142110[/snapback]

I'll be sure to write.

36,406 miles as of this morning. Surely to fail sometime this afternoon...

LaughingMan · 10-17-2005, 11:00 AM

I used to have big variations as much as +7 MPG one tank, and -7 MPG next tank compared to the MFD calculated one.

I never overfill the tank, always let it go on the first click.

Lately the variations have gotten a lot better, probably because I've settled into a routine, and have started going to the same gas station and try to use the same pump every time. Last 3 tanks have been within 1 MPG of the computer.

As long as you stay consistent in the way you fill up at the pump, it should eventualy even out to what the computer tells you.

And for the reason that John posted above, I'm not going to overfill the tank. I think that emissions and the health of the car are more important than outright range. I put in 8-9 gallons every time, and only get about 450-480 miles to a tank, but that's fine by me.

tomdeimos · 10-17-2005, 11:33 AM

Like MikePaul, I try to fill my tank full, at least when I have time.
But I generally stop when the noise changes.

If the car is designed right it shouuld never fill the canister. There is a valve to prevent that in the new tank fix.

Anyway, there should be no risk at all if you stop at the noise change, which should be where the gas gets to the top of the tank and just starts going up into the filler tube.

Even when I got it up to wher I saw gas in the neck my scanner has never reported a purge, so I have to assume my tank valve is working just fine and protecting the sensors.

10-14-2005, 03:35 PM	#5
mikepaul Senior Member Join Date: Dec 2003 Location: Columbia, SC Posts: 1,675 My Car: 2004 Prius Package: #9 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	This goes against all things Green, but... I cram the tank full of gas in order to be sure it's full. I stop when I see gas left near the top. I then drive until my MFD display shows I've driven 10 times the current MPG average, then fill up again, hopefully using 10 gallons. I've never varied more than a half-gallon when I do this, and it's always been more gas than I expected to pump, not less. Given bladder stiffness and other variables, I don't think I'm ever off more than 5%. As I've got no reason to keep a journal of MPGs, I only check this occasionally. When it's almost dead-on accurate, which is often, I'm happy... __________________ 2004 Seaside #9/BC as of 1/3/2004 Oil now any GOOD synthetic I can get on sale Goodyear TripleTred 195/65R15 tires since 5/19/05 80,000 miles as of 8/22/2008

10-17-2005, 11:00 AM	#9
LaughingMan Senior Member Join Date: May 2005 Location: Marlborough, MA Posts: 1,386 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	I used to have big variations as much as +7 MPG one tank, and -7 MPG next tank compared to the MFD calculated one. I never overfill the tank, always let it go on the first click. Lately the variations have gotten a lot better, probably because I've settled into a routine, and have started going to the same gas station and try to use the same pump every time. Last 3 tanks have been within 1 MPG of the computer. As long as you stay consistent in the way you fill up at the pump, it should eventualy even out to what the computer tells you. And for the reason that John posted above, I'm not going to overfill the tank. I think that emissions and the health of the car are more important than outright range. I put in 8-9 gallons every time, and only get about 450-480 miles to a tank, but that's fine by me. __________________ "I thought what I'd do was I'd pretend I was one of those deaf mutes... or should I?" 2005 Seaside Pearl Prius, with Nav. "Tachikoma"

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10-13-2005, 05:38 PM	#1
ddockery Junior Member Join Date: Oct 2005 Posts: 3 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	I've had my Prius since early Feb and am at just over 10k miles and just had my 2nd oil change. Lately, I've been doing my own milage calculations. Over 100 miles, it reported the MPG at ~40 when it was actually 50. I tried it again on a 300 mile trip and it reported 49MPG when I really got 39. Is this type of fluctuation normal? I don't expect it to be perfect, but this seems to be off by more than should be typical. Thanks, Drew

10-13-2005, 05:58 PM	#2
orangeblood Join Date: Jul 2005 Location: Woodbridge, VA Posts: 29 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	The big variable is how much gas you put into the tank. You may have read about the bladder the Prius has for a "tank," and the varying amounts of gas you can get into it each time. If it happens to click off early when there is still room for another couple of gallons in the tank, your mileage will look better than it should. Likewise, if you are able to squeeze more than normal into the tank your calculated mileage will be reduced. Over time these variations should even out, but I trust the computer-generated MPG figure on a tank-by-tank basis. OB

10-13-2005, 05:58 PM	#3
9G-man Senior Member Join Date: Jul 2005 Location: Atlanta, GA Posts: 784 My Car: 2007 Prius Package: #5 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Could you clarify how you are determining actual MPG at the 100 and 300 mile intervals?

10-14-2005, 02:34 PM	#4
DanMan32 Join Date: Aug 2004 Location: Tampa Bay, FL Posts: 3,775 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Lets assume for a moment that the MFD is accurate. For your 100 mile trip, the MFD reports that you used 2.5 gallons (100/40) where you reported using 2 gallons. For your 300 mile trip, your MFD reports 6.12 gallons used while you report 7.69 gallons. But lets add them up. MFD says you used 8.62 gallons while you filled a total of 9.6 gallons. It is quite possible you could have a difference in fill by as much as a gallon.

10-17-2005, 09:52 AM	#6
ddockery Junior Member Join Date: Oct 2005 Posts: 3 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	I didn't realize that filling the tank could very by that much. That's probably making the difference. Thanks for the heads up.

10-17-2005, 11:33 AM	#10
tomdeimos Join Date: Feb 2004 Location: Lexington, MA Posts: 995 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Like MikePaul, I try to fill my tank full, at least when I have time. But I generally stop when the noise changes. If the car is designed right it shouuld never fill the canister. There is a valve to prevent that in the new tank fix. Anyway, there should be no risk at all if you stop at the noise change, which should be where the gas gets to the top of the tank and just starts going up into the filler tube. Even when I got it up to wher I saw gas in the neck my scanner has never reported a purge, so I have to assume my tank valve is working just fine and protecting the sensors.