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Discussion Starter · #1 ·
Has anyone been able to re calibrate the speedometer on a T3 after adding larger tires ? Mine is off a bit I suspect after adding 32's. GPS says about 5mph at 55, but I am suspect of the accuracy of my phones gps.
 

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2011 VR6 FSI LUX
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I recall someone here once posting that the Tregs speedo is off by 3%... I think on the lower side.
I think your phones GPS is more accurate. With 32's you might actually be spot on.
 

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Discussion Starter · #4 ·
I like the out of the box thinking there... 🤣
I thought I read somewhere that civilian GPS was limited to down to about 10ft accuracy. Not sure if that is true, but wouldn't that affect speed tracking?
 

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Keep in mind speedo accuracy and odo accuracy are 2 different things
 

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Keep in mind speedo accuracy and odo accuracy are 2 different things
I keep seeing this. What are the two inputs? If you change tire diameter, both should change, right? I'm pretty sure the odo isn't from a GPS so any % change in speedo should result in an equal % change in odo. This forum says speedo is calibrated to read fast so you don't speed - like anyone wouldn't figure that out in about two days and just drive faster - but odo is correct. So that means the logic in the odo has a fudge factor built in. The fudge factor isn't fluid - it doesn't know the tire diameter is different so it still calculates the same way, which is apparently different than the speedo.
 

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I keep seeing this. What are the two inputs? If you change tire diameter, both should change, right? I'm pretty sure the odo isn't from a GPS so any % change in speedo should result in an equal % change in odo. This forum says speedo is calibrated to read fast so you don't speed - like anyone wouldn't figure that out in about two days and just drive faster - but odo is correct. So that means the logic in the odo has a fudge factor built in. The fudge factor isn't fluid - it doesn't know the tire diameter is different so it still calculates the same way, which is apparently different than the speedo.
You are absolutely correct, my point is the speedometer on your diameter are still going to read a percentage different from accurate
 

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So what are the inputs for speedo and odo? I could see one coming off the transmission and going to two different places, each with their own calibration factor.
 

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So what are the inputs for speedo and odo? I could see one coming off the transmission and going to two different places, each with their own calibration factor.
It's all electronic and a function of equations I think. The rest is beyond me my man.
 
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I don't believe the "source" of the input differs.
I'd suspect that it comes from the ABS module\coding and that it reads off the wheel speed sensors.
This reading is more than likely interpreted by the ECU in conjunction with other factors, and then "presented" to the driver by speedo readout. The speed is "inflated" as per German\Euro law\requirement to "assist" people in NOT speeding..... however, the actual milage\distance\ODO reading is not altered as if it were, it would be fraud.

Should be pretty easy to validate with a late model\high end GPS device if you are really that worried about speedo readout accuracy.
 

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I was wondering if it used wheel speed sensors. That is another fairly obvious source. My first new motorcycle (1995 model year) took the speed reading off the output shaft of the transmission and fed it to the cluster so there was no speedo cable coming off the front wheel. The Trailblazer I had used the front wheel speed sensors and the transmission output shaft as its three inputs for ABS and traction control. The speedo input was solely from the transmission. Probably from the good old days of using a gear-driven cable on the tail shaft.

I am good with a phone GPS telling me I'm reading 0.8 mph faster than GPS speed. The larger tires have me almost matching so I just accept the speedo as correct.
 

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My T3 Exec is slow by about 3mph above 70mph (ie speedometer says 70 but GPS says 73). It seems to me that it's setup based on the 18" or 19" wheel size of the Sport and Lux models instead of the 20" wheels on the Exec. My old Sportwagen TDI had an MDI setting to change the wheel size to adjust the speedometer, but the Touareg doesn't have that setting. So can you adjust this with VCDS?
 

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Cannot adjust that
 

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That over-read of actual speed is as it should be and has nothing to do with the OEM 17/18/19/20/21 inch wheel and tire packages since they all run out at 29 inches overall diameter give or take a fraction.
I disagree that it's as it should be, again my speedometer reports a slower than actual speed. Actual speed is 73, speedometer says I'm going 70. I'd agree if it were reversed. I should also mention that the discrepancy increases with speed, which is why I think it's computing the speed with the wrong size tires/wheels.
 
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