Innacurate Odometer, but accurate Speedometer?

Stuffing

Active member
Joined
Dec 13, 2017
Location
Seattle
TDI
2003 Golf TDI ALH
Hey guys. This last weekend I drove to and from Portland, and noticed something I hadn't seen before with my (relatively) shorter trips.
The first thing I noticed is that my odometer trip reading was far off from what I was expecting. My Google Maps trip length was 167 miles, but my odometer only read 149, with 10 or so miles on there from before. I also compared the trip odometer readings to mile markers though I'm not sure those can really be trusted for any degree of accuracy; my trip reading was generally 85 to 90% of a mile between each marker. Using a GPS based speed app on my phone, my speedometer is reading within a couple percent of my 'actual' speed. My tires and rims are stock sized.

I'm wondering where the inaccuracy is coming from. How accurate are the odometers in these cars generally? Is it possible my 1.8T gauge cluster is screwing with this somehow? (That gosh darn previous owner... :mad:)

I wouldn't be all that worried about this except that I want to be good with scheduled maintenance. If I'm going to be replacing my timing belt a little before the next 100,000 miles, then based on the odometer reading there might actually be more like 115,000 miles on my belt... It's also of course nice to see how my FE is doing.
 

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
What size wheel/tire combination art you currently running?
Typically the MklV cars with stock size wheels/tires run with the speedo reading being 3-4 mph higher than actual while the odometer reads very close to actual (within 1-2%). Having owned six or seven Jetta sedans/wagons, they all have been similar until I changed up tire sizes.
Suggest an actual gps trip to document speed actual vs indicated and miles traveled over sufficient distance to be reasonably accurate.
 
Last edited:

ToxicDoc

Veteran Member
Joined
Mar 1, 2018
Location
Virginia, US
TDI
2001 Jetta, S7, .216
x2. check tire size. MkIV notorious for correct odometer but speedometer reads too fast. I've read some years of Miatas have similar issue.
 

mrfiat

Veteran Member
Joined
Mar 24, 2008
Location
Los Ranchos, NM
TDI
2003 Jetta TDI (Reflex Silver) , 2003 Jetta TDI Wagon (Black)
I have found with 15" wheels the MK4's odometer loses 1.5 mile per 100 on the odometer. The speedo usually reads a bit low. When calculating FE, just add the 1.5 miles per 100 back in.
 

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
I have found with 15" wheels the MK4's odometer loses 1.5 mile per 100 on the odometer. The speedo usually reads a bit low. When calculating FE, just add the 1.5 miles per 100 back in.
Wheel size isn't as relative as tire size/circumference. There are lots of tires that fit the wheels. The stock 195/65/15 tires changed to 205/65/15 (depends on brand and "model") bring the speedo quite close to actual and the odometer to register a bit under or very close to actual miles traveled.
 

Stuffing

Active member
Joined
Dec 13, 2017
Location
Seattle
TDI
2003 Golf TDI ALH
I have the stock steelies with the stock tire size (195/65/15).

I'll do some more testing with GPS indicated speeds, but whenever I pull it out it's reasonably accurate, reading no more than 2 or 3 MPH above or below what my speedometer indicates at 65 MPH. That much of a difference shouldn't account for an odometer reading 85% of 'actual' mileage.

I'll also start doing google maps trips so I can compare what the mileage increase SHOULD be compared to what the trip odometer reads. I'll get back to you guys with those results.

Given that my gauge cluster was swapped in the past (by a shop at least, and supposedly for broken LEDs so I sort of trust the mileage) my suspicions mostly lie there.
 

Stuffing

Active member
Joined
Dec 13, 2017
Location
Seattle
TDI
2003 Golf TDI ALH
And also for a little more information: the gauge cluster got swapped to a 7k redline, 160mph cluster. The tachometer always reads higher than actual (Needle sits at 1100RPM at idle), but the speedometer reads pretty accurately. (Anyone in Seattle have a TDI cluster I can buy...? I need to rent a VAG-Tacho anyway to get some extra keys programmed)

As for why the previous owner (or their shop) thought some LEDs were more important than a good RPM reading and glow plug lights in a manual TDI, I am unable to answer that.
 

mrfiat

Veteran Member
Joined
Mar 24, 2008
Location
Los Ranchos, NM
TDI
2003 Jetta TDI (Reflex Silver) , 2003 Jetta TDI Wagon (Black)
Of course different size tires than stock would affect the odometer. I was referring to the stock tire size with 15" wheels.
 

drucifer

Veteran Member
Joined
Dec 21, 2013
Location
fredericksburg virginia
TDI
2004 jetta sw tdi pd
Could be that someone messed with the Odometer/speedometer using VCDS too...?
As far as I know your odometer i's driven by a tone ring on one or more hubs. Have all codes scanned with vcds. There might be something in the abs. Could a dirty tone ring not be giving enough pulses per rotation?
 

Stuffing

Active member
Joined
Dec 13, 2017
Location
Seattle
TDI
2003 Golf TDI ALH
Alright, I have some data. Each drive I took I compared to the same route on Google Maps. Here's a few trips:

Trip 1
Google Maps: 5.8 Miles
Odometer: 4.9 Miles
84.5% of actual

Trip 2:
Google Maps: 18.8 Miles
Odometer: 16 Miles
85.1% of actual

Trip 3:
Google Maps: 20.2 Miles
Odometer: 17.1 Miles
84.65% of actual

I also tested my speedometer again on the freeway, cruise control at 65MPH, GPS speed never went lower than 64MPH and never went higher than 67MPH. The difference in mileage is so consistent, something is definitely going on I think... This of course assumes Google Maps is reasonably accurate, and at least personally I think that it is.

As far as I know your odometer i's driven by a tone ring on one or more hubs. Have all codes scanned with vcds. There might be something in the abs. Could a dirty tone ring not be giving enough pulses per rotation?
I'll run VCDS again and see if I find anything, never seen an ABS code on there before though. I suppose it could have happened recently.
 

cleaver

Veteran Member
Joined
May 8, 2006
Location
Berwick, Nova Scotia
TDI
None - did own '01 and '02 Jetta TDI
I would think Google Maps should be pretty accurate, but if you have a GPS to track you speed, can't you use your GPS to log your miles driven?

If you aren't using the stock cluster that came in your car originally, are you using the stock ECU? Something is definitely off if Google Maps is accurate.
 

mrfiat

Veteran Member
Joined
Mar 24, 2008
Location
Los Ranchos, NM
TDI
2003 Jetta TDI (Reflex Silver) , 2003 Jetta TDI Wagon (Black)
I just get on the highway and use the mile markers to see how accurate the odometer is.
 

Stuffing

Active member
Joined
Dec 13, 2017
Location
Seattle
TDI
2003 Golf TDI ALH
I just get on the highway and use the mile markers to see how accurate the odometer is.
I did this too - the odometer would consistently roll up by about .85 to .9 miles between each mile marker.
Could be that someone messed with the Odometer/speedometer using VCDS too...?
I think this might be the one...
Based on the fact that the gauge cluster does not match the signal coming to the tachometer with its actual labeling (thus reading 1100RPM at idle) it then would make sense that the same is true for the Speedometer. So essentially, if we're right, and if you did nothing except just install a 7k RPM 160mph cluster into your TDI, the speedometer would ALSO read incorrectly. If you compare the 160mph and 140mph clusters, 140MPH is 87.5% of 160MPH, which matches closely with the discrepancy I'm seeing.

So, the shop that swapped this cluster in may have also cranked the speed signal down in VCDS so that it matched with the new speedometer. Then, the odometer would read the turned-down speed, giving lower miles.

But then, this assumes a lot about the programming and wiring of this stuff... We would need to assume:

  • VCDS is actually capable of turning your speed signal down to this degree.
  • Speed signal must be adapted to read correctly when a higher speed cluster is installed.
  • The signal that reaches the odometer is the same across different clusters, meaning if we hadn't adapted the speed down in VCDS then the odometer would read correctly and the speedometer incorrectly.
Does this logic make sense...?
 

Stuffing

Active member
Joined
Dec 13, 2017
Location
Seattle
TDI
2003 Golf TDI ALH
Finally got the TDI Cluster in. Did a speed test before reprogramming the Cluster Coding for speed, and as I suspected it was reading very low. It was reading around 30MPH at 35MPH. I corrected the coding in VCDS as shown here, and while it may need some more tweaking it's pretty accurate now and the odometer is rolling up much more accurately as well.

Now I just need to get my radio code and get my radio out of safe mode :rolleyes:
 

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
Finally got the TDI Cluster in. Did a speed test before reprogramming the Cluster Coding for speed, and as I suspected it was reading very low. It was reading around 30MPH at 35MPH. I corrected the coding in VCDS as shown here, and while it may need some more tweaking it's pretty accurate now and the odometer is rolling up much more accurately as well.

Now I just need to get my radio code and get my radio out of safe mode :rolleyes:
I've gotten a few radio codes from dealers in the past (the owner's manual says they will supply them at n/c, but last reports I read was most stealerships were charging to provide them). All you need is the radio serial number although most will want the vehicle VIN also, but that's just them making sure you own that vehicle I think.
 
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