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1,011 Posts
Podex do you mean the new firmware update? Or does mine do something I didn't know it did?
You have the x-gauge one, right? If so, you can change speed calibration as a percentage of the ECU speed. Looking at the instructions, go to MORE>SETUP>SPEED and change the percentage to match actual speed.

Here it is from the manual (copied and pasted, so it's not my fault if it's confusing)

Both speed and distance can be adjusted to compensate for changes in
tire size, gears, tire wear, etc..

The setting can be changed at any time. If SAVE is not used, the entered
value will be lost when the ScanGaugeII is disconnected. This may be
the desired operation if the vehicle is not to have the ScanGaugeII
regularly used on it.

The right and left upper buttons can be used to increase/decrease the
selected adjustment in 1% steps. The lower left number is the speed
reported by the vehicle. The lower right number is the speed which will
be shown by the ScanGaugeII by applying this correction. Positive
values of % will increase the displayed speed. Negative values of % will
decrease the displayed speed. The approximate % to use for a tire size
change can be computed by dividing the diameter of the original
equipment tire by the diameter of the new tire, subtract this from 1 and
multiply by 100. For instance, if the diameter of the original tires was 24
inches and the size of the new tire is 30 inches, 100x(1-(24/30)) = 20.
Setting the value to 20% will compensate for the tire size change.
For gear changes, the formula would be 100x(1-(new ratio/old ratio)).
For instance if the old ratio was 3.55 and the new ratio was 4.10,
100x(1-(4.10/3.55)) = -15%.

3,712 Posts
I use the Speedometer function on my Passport 9500i and the speedo function on my Kenwood 8120. Even with 285s my speedo is off by about five MPH at highway speeds. The passport and the nav unit always read the exact same speed by the way. It works great because I can't see the speedometer because of my seat/wheel position anyhow.
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