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If you are using Google Maps, this might be possible using the Google Map Api. See Check your speed - Google Maps Help . The roadway Google Map of your area must have the feature to use the Speedometer. Use the Roads API with a Google Map. When you are successful, please let us know. You will have to access Google Map using the Static Map method probably. Some builders can access Google Maps using a control; App Inventor cannot and must rely on an api and the WebViewer.
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if you are using the Map component (OpenStreetMaps) I do not believe this is possible. A clunky work around might be possible using the camera constantly viewing Speed Limit signs. Something like Local image searching bar (Classify or read the contents of road signs)