Odometer Accuracy

I had a Search and rescue meeting tonight. On my way home I thought about how accurate my odometer is. Since I had the GPS on the dash and the ScanGauge II. I decided to clear the trip on the odometer, clear the GPS odometer and then clear the ScanGauge II odometer. Also the ScanGauge is set with a 1% offset to correct the speed to correct with the GPS. I know the distance for the test was rather short but it did uncover a odometer error on the stock cluster. The test distance was 4.95 miles exactly on the GPS. The ScanGauge II was showing 4.9 miles and the truck odometer was showing 5.0 miles. I'm going to re-run this test again on a long test on post up my findings.

This was kind of one of those in the back of my head thought to do just to see what would happen. But from what I've seen so far my GPS and ScanGauge II are right on for distance but the truck is off. I'm going to try for a 100 mile trip and see what happens.

-- More to come.

This means you probably don't have 200k miles on your truck then. Sorry.

This means you probably don't have 200k miles on your truck then. Sorry.

I'm just curious. what I'm actually up to is figuring out a test bed method of checking for odometer accuracy on either the truck or ScanGauge. So I'm more or less thinking out loud and keeping my notes here.

Until I put the smarty on to correct the speedo, I noticed a significant error (>5%) in my Odo. Since then, its spot on.

I have issues with the GPS because it is a satellite looking straight down at you so if per say you were going straight up, it would see that as no movement. I never see the elevation on it being all that great either so I am not sure how accurate it really is on a hilly road. I'm sure it is supposed to calculate for elevation changes but I'm just not convinced it's all that dead nuts. I don't know how accurate the markers on the interstate are but I would like to think those are the most accurate thing to go by.

I have issues with the GPS because it is a satellite looking straight down at you so if per say you were going straight up, it would see that as no movement. I never see the elevation on it being all that great either so I am not sure how accurate it really is on a hilly road. I'm sure it is supposed to calculate for elevation changes but I'm just not convinced it's all that dead nuts. I don't know how accurate the markers on the interstate are but I would like to think those are the most accurate thing to go by.

GPS triangulates using elevation as well. Its pretty accurate. My handheld garmin is usually within 3-5 ft of the surveyors super expensive error corrected GPS units.

Well here is run number two. I had to run down to Fire Station #3 this morning to drive a fire truck to Grangeville, ID. So I ran the test again for my truck from home to the fire shed and back to home. Garmin GPS V ...... 21.0 miles ScanGauge II ....... 21.0 miles Truck Odometer .... 21.6 miles Seem like my truck is truly 0.5 MPH as seen on the Garmin GPS V I've got. https://buy.garmin.com/en-US/US/prod155.html

. I don't know how accurate the markers on the interstate are but I would like to think those are the most accurate thing to go by.

From what I have noticed they are not all that accurate, unless they are the ones marked as a speedo calibration area.

Well here is run number two. I had to run down to Fire Station #3 this morning to drive a fire truck to Grangeville, ID. So I ran the test again for my truck from home to the fire shed and back to home. Garmin GPS V ...... 21.0 miles ScanGauge II ....... 21.0 miles Truck Odometer .... 21.6 miles Seem like my truck is truly 0.5 MPH as seen on the Garmin GPS V I've got. https://buy.garmin.com/en-US/US/prod155.html

My OEM tire setting with OEM tires was off by 4-5% (don't recall exactly), but since I have put the Smarty on its about .5-1% slow (how I like it so I am not racking up more miles than actually driven). Something else I have noticed on all 3 of my vehicles is the indicated speed is not calibrated to match the odometer. My VW Jetta is the worst where it reads about 2 mph fast at 55, but the odometer is about 1% slow. The Dodge and Toyota are differenent from speedo to odo as well. Basically the odometer is what should be calibrated, and not the speedo. I, like CSM, have found my Garmin to be quite accurate but when I am trying to adjust my speedometer I don't like doing it on anything but interstate travel out of the mountains. My speedo and GPS will match until I start hitting winding roads thru mountains areas (I think you know what those look like, I presume you have driven on 55 ). I have also found my hiking GPS to be sligthly more accurate and consistant than my vehicle GPS when they are both mounted side-by-side on the dash. Several years ago on a previous set of tires I had the speedo pretty well dialed in, after 1000 miles on the GPS the truck said 997. Two last things to consider. Tire pressure plays a big role. If I keep my tires at 80 psi with the truck empty the speedo is off, but it's accurate when loaded up to 5K plus on the rear axle. When I drop the empty pressure to 45-50 it's back to being accurate. The speedo may be 1% off at 30 and 1.5% off at 80. I know that is really splitting hairs, but hey what the heck. So, is a Smarty in your future to adjust the speedo?

There is a typo in my last post.

Garmin GPS V ...... 21.0 miles ScanGauge II ....... 21.0 miles Truck Odometer .... 21.6 miles

Corrected number I was trying to remember and post it and screwed it up. :banghead: Garmin GPS V ...... 21.6 miles ScanGauge II ....... 21.6 miles Truck Odometer .... 22.0 miles Ok... Bit long extended trip added into today till I quit... Garmin GPS V ...... 76.4 miles ScanGauge II ....... 76.5 miles (+ 0.1 Off) Truck Odometer .... 77.9 miles (+ 1.4 off)

So, is a Smarty in your future to adjust the speedo?

No. Already got correct speedometer for the most part. But this also proves my fuel data from the ScanGauge II is very close to accurate. (Formula: MPH / GPH = MPG)

Something else I have noticed on all 3 of my vehicles is the indicated speed is not calibrated to match the odometer. My VW Jetta is the worst where it reads about 2 mph fast at 55, but the odometer is about 1% slow. The Dodge and Toyota are differenent from speedo to odo as well.

Between my 02 on 96 Dodges the 02 is off 0.5 MPH and then the 96 is a special case. The 96 shows 4 MPH fast on the needle but the OBDII port is right on the money.

Truck Odometer .... 77.9 miles (+ 1.4 off)

2% isn't bad by OEM standards, but I like mine closer. At 200K you only have 196K. While it's not enough to mean anything I personally don't like it.

No. Already got correct speedometer for the most part. But this also proves my fuel data from the ScanGauge II is very close to accurate. (Formula: MPH / GPH = MPG)

Yeah, it's nice to be able to adjust the SGII for mpg accuracy.

Between my 02 on 96 Dodges the 02 is off 0.5 MPH and then the 96 is a special case. The 96 shows 4 MPH fast on the needle but the OBDII port is right on the money.

That's similar to my VW. I think it's a design feature.

That's similar to my VW. I think it's a design feature.

John, I have a VW Jetta 2006 TDI and the odometer was off by 10% at any speed. I had the Boise dealership input a correction and now it is within the +/- 3mph spec. Actually it is +3mph. My wife's 2012 Jetta TDI is right on the money from the factory.

My odo is within 1%, it's just the needle that is off.

My odo is within 1%, it's just the needle that is off.

I found the work order. Performed coding change from 7203 to 7207 in value Block 17 for instrument cluster for speedometer inaccuracy. Speedometer is 3mph plus which is within VW specs. They should do this for free.

Does that effect the odo as well?

Does that effect the odo as well?

I honestly do not know. I purchased the car 1 year old and in warranty, so it was the first thing to catch my eye. Then my wife bought another 2006 TDI and same issue was found, so they corrected it. We took a vacation in her car and the navigation system was off by 10% in mileage. Dealership told me to contact NavTech about the navigation mileage issue, but I think they did not want to invest time in detailed work. Then it was totaled and she bought the 2012 TDI which the speedometer is perfect, but we have not taken a vacation in it yet to view the odometer.

Usually when you adjust the speedo the odo adjusts too. If I were to make my speedo accurate my odo would read fast, putting more miles on the car than actual. I don't want to have that happen.

I don't know about the odo... but my speedometer is dead nuts on... closer than I can measure, based on the GPS. And this is with the OEM rubber that I still have on there. Typically speedometers read slightly fast so purchasers don't rack up speeding tickets. I was always annoyed when convoying with wife, that she was always dogging it... but HER Hundai speedometer read slow.

My Smarty lets me adjust the speedo on my 05, I adjust the odo and take the small (~1%) speedo error into account.

Usually when you adjust the speedo the odo adjusts too. If I were to make my speedo accurate my odo would read fast, putting more miles on the car than actual. I don't want to have that happen.

I found the instrument cluster of a VW Jetta (from May 2005) the speedometer(G21) and tachometer (G5) receive data from the instrument cluster control module. There are sensors on each wheel for ESP control and ABS. But the odometer might come from the transmission control module which makes sense for automatic or DSG but not manual.