No Idea What My Actual Mileage Is

TooFolkGR · 10-12-2007, 04:17 PM

Well it's like this... according to my trusty GPS (which I suppose could be wrong) my speedometer is 2-3 MPH fast. I assume since the MPH are wrong, the odometer is wrong too. If the odometer is wrong, the mileage is wrong. I average about 45.4 in my (mostly freeway) driving.

I've tried calculating the mileage manually, but that's completely impossible since I can't put the same amount of gas in my tank any two times because of the bladder.

I know I'm saving a lot of money on gas over any car I've ever owned, and I'm overall happy. But it's a little disappointing that I don't know exactly what my mileage is. Overall I expect since the speed is too high, the mileage is probably too high too so it's just as well that I don't know.

I'd be disappointed if I were suddenly sure and it was like 41 MPG.

Rae Vynn · 10-12-2007, 04:20 PM

What you do, is you keep a logbook of every fillup, and all mileage.
After 5 or 10 tanks, do the calculation manually.
Your average is what is important, not each individual tank.

Ichabod · 10-12-2007, 04:51 PM

And no car can be exact in the sense that you described because of such things as tire pressure throwing the numbers off by a little.

It's close enough for government work, but if you want you can do a measured mile at constant speed (i.e. set cruise at 60 and find yourself a measured mile marker on the highway.. time it, and the difference between your time and one minute is your fudge factor) every fillup, or once a month, or twice a year, or whatever makes you feel reasonably confident that you're getting a closer estimate. Then jot that down with the spreadsheet that Rae Vynn recommended, and you can adjust your numbers to give you a closer estimate.

Personally, I don't do it because I can reasonably guess at the difference, and the difference doesn't mean much to me anyway. As long as I'm setting personal-best-tank-avgs and improving lifetime FE, I'm happy

richard schumacher · 10-13-2007, 12:13 PM

In European cars the speedo deliberately reads high, to unconsciously discourage speeding. Maybe this is done a little with US cars also?

But the speedo and the odometer are independent. If your tires are the right size then I'd bet that the odometer is correct.

As for MPGs, the readout on the MFD is accurate enough.

mcbrunnhilde · 10-13-2007, 01:26 PM

I have tracked my fillups ever since I bought my car 3-1/2 years ago. I also "reverse engineered" my display mileage to show lifetime stats (the 2004 completely resets after each tank). The display lifetime mileage is consistently about 2% higher than what I calculate with the gas I put in the car. At 50 mpg, that's only 1 mpg off...

Bob64 · 10-13-2007, 03:03 PM

Non-military GPS devices are accurate only to 3-10 meters... depending on the model...

PriuStorm · 10-13-2007, 08:28 PM

<div class='quotetop'>QUOTE(Ichabod @ Oct 12 2007, 03:51 PM) [snapback]524814[/snapback]</div>

Quote:

And no car can be exact in the sense that you described because of such things as tire pressure throwing the numbers off by a little.

It's close enough for government work, but if you want you can do a measured mile at constant speed (i.e. set cruise at 60 and find yourself a measured mile marker on the highway.. time it, and the difference between your time and one minute is your fudge factor) every fillup, or once a month, or twice a year, or whatever makes you feel reasonably confident that you're getting a closer estimate. Then jot that down with the spreadsheet that Rae Vynn recommended, and you can adjust your numbers to give you a closer estimate.

Personally, I don't do it because I can reasonably guess at the difference, and the difference doesn't mean much to me anyway. As long as I'm setting personal-best-tank-avgs and improving lifetime FE, I'm happy

[/b]

I believe this to be the case as well here in the U.S., that the speedometers read optimistic by several mph. On the Suzuki Burgman site (a maxi scooter), it is a frequent bone of contention that the speedometers intentionally and consistently read on average 10% optimistic... i.e. 70mph indicated is only 63mph actual. Be happy that you're only off by a couple of mph...

DaveinOlyWA · 10-13-2007, 09:01 PM

my GPS always registers the speed as being on average a mile or two lower than Prius speed. but my GPS also registers in 3D so going up and down hills will dramatically change the readings as well, so trying to figure mileage from GPS is not really practical.

one thing i have noticed is that even at much higher speeds, the difference does not seem to increase. now the speed on the Prius is determined by rotations of the wheel. since a true speed involves the rate traveled past a fixed point, a straight line is assumed. i for one, will say my driving is not straight. albeit, the variance is probably small, but GPS also does straight line calculations and GPS is not even advertised as 100% accurate, so if i were you. i would just take what your Prius says... it may not be perfect, but lets face it, nothing you have access to is either.

patsparks · 10-14-2007, 11:00 AM

Speedo and odometer acuracy aren't nessesarily connected. My bike speedo is out by 11% but the odometer is within 1% over a measured distance. I can't be sure of all markets but here vehicle standards require speedos to read from 1% to 10% fast.

JimboJones · 10-14-2007, 11:21 AM

<div class='quotetop'>QUOTE(patsparks @ Oct 14 2007, 11:00 AM) [snapback]525420[/snapback]</div>

Quote:

I can't be sure of all markets but here vehicle standards require speedos to read from 1% to 10% fast.
[/b]

In Australia the law states that speedo errors need to be within PLUS or MINUS 10% actual. This means speedometer readings can be faster or slower than actual.

http://www.aussiemotorists.com/misc/MSACon...ELIX2_2004.html

10-12-2007, 04:51 PM	#3
Ichabod Artist In Residence Join Date: Jul 2006 Location: Newton, MA Posts: 1,774 My Car: 2006 Prius Package: #7 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 7	And no car can be exact in the sense that you described because of such things as tire pressure throwing the numbers off by a little. It's close enough for government work, but if you want you can do a measured mile at constant speed (i.e. set cruise at 60 and find yourself a measured mile marker on the highway.. time it, and the difference between your time and one minute is your fudge factor) every fillup, or once a month, or twice a year, or whatever makes you feel reasonably confident that you're getting a closer estimate. Then jot that down with the spreadsheet that Rae Vynn recommended, and you can adjust your numbers to give you a closer estimate. Personally, I don't do it because I can reasonably guess at the difference, and the difference doesn't mean much to me anyway. As long as I'm setting personal-best-tank-avgs and improving lifetime FE, I'm happy __________________ 2006 Barcelona Red Metallic - a.k.a. "The Professor" :: Accessories/Mods :: BT Tech Stiffening Plate - Engine Block Heater - OEM EV button - OEM Mud guards - Rear Bumper Protector :: Near-future :: ScangaugeII

10-13-2007, 09:01 PM	#8
DaveinOlyWA Opps !! I Did it Again!! Join Date: Apr 2004 Location: South Puget Sound, WA Posts: 9,633 My Car: 2006 Prius Package: #4 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 5	my GPS always registers the speed as being on average a mile or two lower than Prius speed. but my GPS also registers in 3D so going up and down hills will dramatically change the readings as well, so trying to figure mileage from GPS is not really practical. one thing i have noticed is that even at much higher speeds, the difference does not seem to increase. now the speed on the Prius is determined by rotations of the wheel. since a true speed involves the rate traveled past a fixed point, a straight line is assumed. i for one, will say my driving is not straight. albeit, the variance is probably small, but GPS also does straight line calculations and GPS is not even advertised as 100% accurate, so if i were you. i would just take what your Prius says... it may not be perfect, but lets face it, nothing you have access to is either. __________________ My 2006: Last tank 463.1 miles @49.2 pump (49.4 computer) 4.22 cpm winter mpg 50.70 summer mpg 54.59 lifetime: 33,038.3 miles 52.79 mpg pump (54.04 mpg computer) 5.74 cpm My 2007 Zenn Driven 5945 miles, 2.01 cpm/ 105 mpg (at $2.05/gal), 4.04 miles/ Kwh Savings over my Prius $302.66 The Corolla...573.26 The REAL SAVINGS from not driving Corolla 657.39 +Prius= 958.18 (excludes maintenance costs...would be unfair to ICE vehicles) My Plate: DUALPWR (Dual Power)

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10-12-2007, 04:17 PM	#1
TooFolkGR Member Join Date: Jun 2007 Location: Grand Rapids, MI Posts: 34 My Car: 2007 Prius Package: Base Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Well it's like this... according to my trusty GPS (which I suppose could be wrong) my speedometer is 2-3 MPH fast. I assume since the MPH are wrong, the odometer is wrong too. If the odometer is wrong, the mileage is wrong. I average about 45.4 in my (mostly freeway) driving. I've tried calculating the mileage manually, but that's completely impossible since I can't put the same amount of gas in my tank any two times because of the bladder. I know I'm saving a lot of money on gas over any car I've ever owned, and I'm overall happy. But it's a little disappointing that I don't know exactly what my mileage is. Overall I expect since the speed is too high, the mileage is probably too high too so it's just as well that I don't know. I'd be disappointed if I were suddenly sure and it was like 41 MPG.

10-12-2007, 04:20 PM	#2
Rae Vynn Resident Witch Join Date: May 2007 Location: Grays Harbor, WA, USA Posts: 2,757 My Car: 2007 Prius Package: Base Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 52	What you do, is you keep a logbook of every fillup, and all mileage. After 5 or 10 tanks, do the calculation manually. Your average is what is important, not each individual tank.

10-13-2007, 12:13 PM	#4
richard schumacher nacreous lacquer Join Date: Mar 2004 Location: United States Posts: 4,126 My Car: 2004 Prius Package: #6 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	In European cars the speedo deliberately reads high, to unconsciously discourage speeding. Maybe this is done a little with US cars also? But the speedo and the odometer are independent. If your tires are the right size then I'd bet that the odometer is correct. As for MPGs, the readout on the MFD is accurate enough.

10-13-2007, 01:26 PM	#5
mcbrunnhilde Opera singin' Prius nut! Join Date: Jan 2004 Location: San Diego, CA Posts: 596 My Car: 2004 Prius Package: #7 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	I have tracked my fillups ever since I bought my car 3-1/2 years ago. I also "reverse engineered" my display mileage to show lifetime stats (the 2004 completely resets after each tank). The display lifetime mileage is consistently about 2% higher than what I calculate with the gas I put in the car. At 50 mpg, that's only 1 mpg off...

10-13-2007, 03:03 PM	#6
Bob64 Sapphire of the Blue Sky Join Date: Apr 2007 Location: Virginia Posts: 823 My Car: 2007 Prius Package: #2 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Non-military GPS devices are accurate only to 3-10 meters... depending on the model...

10-14-2007, 11:00 AM	#9
patsparks Uneducated bird-brain Aussie Join Date: Jul 2007 Location: Adelaide South Australia Posts: 4,902 My Car: 2004 Prius Package: Base Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 17	Speedo and odometer acuracy aren't nessesarily connected. My bike speedo is out by 11% but the odometer is within 1% over a measured distance. I can't be sure of all markets but here vehicle standards require speedos to read from 1% to 10% fast.