Speedo correction

Locoelectrician

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2015 Passat TDI SEL, 2015 Golf SEL Sportwagen, 2005 Jetta TDI, John Deere 355D
I traded in my Passat a few weeks ago but before it went in we swapped the wheels/tires from the Passat to the GSW. She never liked how low profile the tires were on the GSW and how rough it rode. Anyways, the GSW SEL now has the Passat SEL wheels and tires and she loves them except that her speedometer is now about 5mph LOW. I’ve searched until my eyes hurt and find a few people talking about going with smaller tires but not ever anyone going with bigger. I have VCDS and have done plenty of coding, I just need to know if it’s possible to correct this? Some people say impossible but those were older threads. So can it be adjusted or is she stuck? Thanks in advance.
 

1854sailor

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I don't think that it is possible with VCDS. The software for the speedo is is in the cluster, so you would have to have the capability to download it, then modify it, then reload it. How much larger are those tires than standard size for the speedo to be reading that low? Ours, on 205-55/16 tires, reads ~2 MPH high.
 

Locoelectrician

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I forget the size difference but both cars are SEL so they have the 18s. Hers were pretty low profile compared to mine though.
 

Locoelectrician

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Jetta_Pilot

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Locoelectrician, how are you determining your speed?
VW's speedometer always read higher than real speed as checked with a GPS on a level straight road. It's 5% higher at km or 2 1/2 % at miles. Your odometer will be correct though.
I always change up one tire size from OEM which brings speed indicated by the speedometer in line with a GPS. But it does change the odometer slightly.
 

Locoelectrician

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Locoelectrician, how are you determining your speed?
VW's speedometer always read higher than real speed as checked with a GPS on a level straight road. It's 5% higher at km or 2 1/2 % at miles. Your odometer will be correct though.
I always change up one tire size from OEM which brings speed indicated by the speedometer in line with a GPS. But it does change the odometer slightly.
GPS. We knew the tires we were putting on were bigger and that the speedo would read low, just didn’t know how bad it would be. I’m familiar with German cars showing a few high and you just get used to it. Hers is about 5 LOW at 70.
 

RIP TDI

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Just remember that these "impulse distance" adaptions ("Tire Circumference - Variation x") are intended for your scenario of fitting non-standard diameter tires and VW's intention is to correct resulting inaccurate odo readings, not inaccurate speedo readings. Changing the impulse distance value alters both but keeps the relationship between them the same, so that a once-inaccurate speedo will be corrected at the expense of a once-accurate odometer. That may be an acceptable tradeoff for some, but that fact sometimes gets lost in the enthusiastic quest for accurate speedo readings.

The technically correct solution is to correct odometer error resulting from non-standard tire diameters using impulse distance adaption and then to correct residual speedo error by editing the cluster chip. At least this was the scenario for pre-Mk7 models; anecdotal evidence suggests that speedo correction independent of odo correction (i.e, NOT impulse distance adaption) is still not available as a VCDS adaption or long-coding change. I would love to be wrong about that, though.

BTW, if trying to calculate something using tire circumference as a variable, a common mistake is to either mathematically convert published tire diameter to circumference or actually measure circumference with a cloth tape measure. Well, its not a mistake in theory, just in accuracy. The most accurate way to do this (for a new tire only) is to use the published revs per mile spec. and convert this to circumference.
 
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Locoelectrician

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Good point. I’m perfectly happy with it reading a little high as it’s designed. Reading low as it does now sucks though.
 

dubStrom

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27" diameter

Has anyone verified that this tire circumference setting is present and works on the Mk7 GSW? I just got taller tires and I know the speedo is reading a bit low (and racking up more miles than indicated). I want to adjust!

The premise that diameter should be selected by tire "size" is ridiculous though. There is significant variation from tire to tire, especially for mud/snow versions. The options should be different diameters, not tire sizes! In either centimeters or inches, it would be much easier to know what you are getting. Such a scale could even accept fractions, like 25.2", 25.3" (64cm, 64,2cm). No reason not to. You could put it exactly where you want it, even with a slight intentional bias for error if you want it (like 1 mph positive error to be sure you are not speeding sometimes). I am sure that the decision to make by tire size was to make it "easy" for technicians and flunkies like us. We are too dumb to know what a diameter is. Stupid users!!
 
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