How "off" is your odometer

[amsoilguy]

Hi all. The odometer is off by + 1.0 miles, over a 30 mile stretch. Went 31 miles even though the odometer indicated 30 miles. I checked this using the mile markers on the highway. Filled up today and hand calculated according to the shown odometer reading I got 49.2 mpg.(17.137 gallons used) The MFD indicated 51.0. When taking into acount the mileage discrepancy, the with 843.2 miles traveled, would indicate an additional 28.1 miles, for a total of 871.3 miles giving a hand calculated 50.84mpg. How different is you odometer from actual? I have the 18 inch tires on mine, not sure if that makes that much a difference, unless the circumference difference between 17 and 18 is that significant.

At 1/16 on the fuel gauge, still had 1.3 gallons in the tank. That is assuming that the 18.5 gallons is accurate. They really had done work on this guage as it is damned accurate from full to empty....

[gfr1]

Originally Posted by amsoilguy

Hi all. The odometer is off by + 1.0 miles, over a 30 mile stretch. Went 31 miles even though the odometer indicated 30 miles. I checked this using the mile markers on the highway. Filled up today and hand calculated according to the shown odemeter reading I got 49.2 mpg. The MFD indicated 51.0. When taking into acount the mileage discrepancy, the with 843.2 miles traveled, would indicate an additional 28.1 miles, for a total of 871.3 miles giving a hand calculated 50.84mpg. How different is you odometer from actual? I have the 18 inch tires on mine, not sure if that makes that much a difference, unless the circumference difference between 17 and 18 is that significant.

You are on questionable ground here. I'm not aware of posted sureyed five or ten mile calibration locations as there once were. Interstates are surveyed, yes, but road change errors are factored in and are not apparent. When you are at a mile marker, that is the same location along the road on both sides, which ever direction you are traveling. Along a large bend and around hills, you might tavel a different distance traveling the opposite way, but will begin and end up at the same mile markers. They don't re-post when a road change is completed, either, other than the local area. If they did, they would have to re-survey across the remainder of the whole state and move all markers. This type calibration could be a bigger error factor than reading your instruments. Then our figures will end up worthless. Then, with GPS, the ommitted tenths, or whatever the measurement is, are dropped. Another words, 19.99 miles or 19.0 miles will both display "19" (at least on most standard automotive/portable systems). Also, the GPS reads distance and speeds over the surface of the earth. An odometer reads the road surface. In theory, if you had a theoretical triangle on edge and you traveled 10 miles up one side and ten miles down the other, the odometer would read 20 miles, but the GPS would only read ten (across the third side), which would also be reflected in the speed. In a high performance fighter jet, with inertial nav or GPS, when you pull straight up, traveling a few hundred miles per hour, the airspeed shows your velociy through the air, but the inertial nav and GPS systems show "0" movement over the ground, assuming that you remain over a point during the maneuver. If you could actually go vertical up and down, the inertial navigtion and GPS would display the wind vector at the altitude of the reading, as you are being blown downwind. (Been there, done that) Measured distancs are hard to find, as are "roller" calibation stations. AAA used to have some mobile units for this, but I haven't seen them in a few years. And, I always wondered about tire radius issues becaus the rollers caused a pretty good depression in the rolling radius, depending on the tire pressure. I have a $500.+ "Race Logic" GPS instrument for such things (as used by performance people), but haven't measured a distance with it as yet.

[amsoilguy]

Originally Posted by gfr1

You are on questionable ground here. I'm not aware of posted sureyed five or ten mile calibration locations as there once were. Interstates are surveyed, yes, but road change errors are factored in and are not apparent. When you are at a mile marker, that is the same location along the road on both sides, which ever direction you are traveling. Along a large bend and around hills, you might tavel a different distance traveling the opposite way, but will begin and end up at the same mile markers. They don't re-post when a road change is completed, either, other than the local area. If they did, they would have to re-survey across the remainder of the whole state and move all markers. This type calibration could be a bigger error factor than reading your instruments. Then our figures will end up worthless. Then, with GPS, the ommitted tenths, or whatever the measurement is, are dropped. Another words, 19.99 miles or 19.0 miles will both display "19" (at least on most standard automotive/portable systems). Also, the GPS reads distance and speeds over the surface of the earth. An odometer reads the road surface. In theory, if you had a theoretical triangle on edge and you traveled 10 miles up one side and ten miles down the other, the odometer would read 20 miles, but the GPS would only read ten (across the third side), which would also be reflected in the speed. In a high performance fighter jet, with inertial nav or GPS, when you pull straight up, traveling a few hundred miles per hour, the airspeed shows your velociy through the air, but the inertial nav and GPS systems show "0" movement over the ground, assuming that you remain over a point during the maneuver. If you could actually go vertical up and down, the inertial navigtion and GPS would display the wind vector at the altitude of the reading, as you are being blown downwind. (Been there, done that) Measured distancs are hard to find, as are "roller" calibation stations. AAA used to have some mobile units for this, but I haven't seen them in a few years. And, I always wondered about tire radius issues becaus the rollers caused a pretty good depression in the rolling radius, depending on the tire pressure. I have a $500.+ "Race Logic" GPS instrument for such things (as used by performance people), but haven't measured a distance with it as yet.

Point taken. Will check mile markers in oposite direction to see how wrong i am. Daily commute, so doing it all the time.

[domano 68]

After my first tank my mpg gauge was showing a higher mpg than my hand calculation. So I decided to check my odometer. I used a gps app Map My Run. Over 25 miles the odom was off by -0 .4 miles on the morning route. It was off by -0.5 mile on the same 25 mile route this afternoon. (gps showed 25 miles, vw showed 24.6). So this discrepancy does not line up with my mpg gauge being higher than the hand calculated #. Hmmmmmm!

I also noticed odom was showing lesser distance than mile marker posts.

[amsoilguy]

Originally Posted by domano 68

After my first tank my mpg gauge was showing a higher mpg than my hand calculation. So I decided to check my odometer. I used a gps app Map My Run. Over 25 miles the odom was off by -0 .4 miles on the morning route. It was off by -0.5 mile on the same 25 mile route this afternoon. (gps showed 25 miles, vw showed 24.6). So this discrepancy does not line up with my mpg gauge being higher than the hand calculated #. Hmmmmmm!

I also noticed odom was showing lesser distance than mile marker posts.

Domano,
Does or does not? Seems to me that you negative distance 24.6 (VW) vs 25(Actual), fits perfectly. 25/2.5 is a greater number than 24.6/2.5.... So hand (VW Odom) would be less than MFD (GPS Actual) right?

[Knotty]

Over flat ground with cruise control on my speedo reads 65 while my GPS shows 67 MPH. Assuming GPS to be accurate, the speedo and presumably odo are 3% low.

[amsoilguy]

Originally Posted by Knotty

Over flat ground with cruise control on my speedo reads 65 while my GPS shows 67 MPH. Assuming GPS to be accurate, the speedo and presumably odo are 3% low.

That almost matches my 1 mile on 30 exactly. Keep forgetting to do the route the other way.... On the way home then and have better things to think about!!

[Yamaguy]

Originally Posted by Knotty

Over flat ground with cruise control on my speedo reads 65 while my GPS shows 67 MPH. Assuming GPS to be accurate, the speedo and presumably odo are 3% low.

My new Passat Speedo shows just like yours when compared to my Garmin gps.

[damonator]

Mine is the same

[gt4494]

The best answer I have found to this subject is that according to the UN standards
http://www.unece.org/trans/main/wp29/wp29regs21-40.html Look up reference - UNECE 39/00 (Rev 1)

all speedometers must not read less than the actual speed of the vehicle. To ensure compliance I understand VWs are designed with an approx 3 mph error to the good (high).

Since speedometers and odometers are now electronic and not mechanically connected the odometer does not have the same error programmed into it.

If anybody has any better answer please respond.