Calibrate Speedo

11th March 2020

I’ve known for a long time that cars’ speedometers generally over-read by approximately 10%. This never really bothered me until the Google Maps on my smartphone (which I use a lot whilst driving) started displaying my current speed (derived from the smartphone’s GPS). I found it annoying seeing my car’s speedometer showing one speed, for example 30mph but Google Maps showing a lower speed, such as 28mph. That’s an over-reading by the speedo of approximately 7.1%.

I vaguely remembered seeing a setting in VCDS which hinted that some calibration of the car’s speedometer might be possible. I did a little research and it looked like adjusting the "Tire Circumference - Standard" setting was the way to do this.

Control Module: 17 (Instruments)
Coding: Byte 3, bits 0-2 (Tire Circumference - Standard)
Set to: 6

The setting holds a value from 0 to 8 which appears to code a different tyre circumference. The value for my car was originally set to “0”. I wasn’t sure how the code related to tyre circumference, but I decided to just try other values and see what the effect was. Initially I tried to code the maximum value, “8” but this value was not accepted. “7” also was not accepted, however “6” was.

After a short test drive, it was apparent that this had had the desired effect. At an indicated 60mph by the speedometer, Google Maps was now showing 59mph. That’s reduced the over-reading from approximately 7.1% to only 1.7%. Close enough. I can live with that.

MFD indicating current speed of 60mph

Google Maps showing current speed (from GPS)
now only 1.7% less at 59mph

Whether this slows down the rate at which my car’s odometer increases I’m not sure. Does this mean my car with 100,126 miles on the clock has actually travelled 7,109 fewer miles?

I have noticed that it has had an effect on the Recommended Gear Indicator on the MFD. It now recommends that I change up a gear sooner.

Hopefully this will mean I will arrive at my destination not just a little quicker but also maybe after using a little less fuel!

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