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I was gathering data using an android phone with the "Trackmaster Pro" application, teamed with a 10hz GPS beacon. That will get you GPS speed and position and accelerations. The program also spits out a .csv file on-demand which you can load to a program called "RaceChrono2AVI" that will allow you to create an overlay for the in-car video.
Then I would combine the two using the picture-in-picture feature of my video editing software (Sony Vegas, lately). The toughest part is getting the videos synced up properly.
There is also a very good app for iOS called "Harry's Lap Timer" that will gather data as well, but I'm not sure if it can spit out the .csv file to create an overlay. HOWEVER if you're taking video with your iOS phone in addition to gathering the data, HLT will export a video with the overlay already embedded for an entire session or specific lap/laps. Saves a ton of time editing etc, can crank out a single lap video with telemetry in under 5 mins. The downside is that I have no idea how to marry it to external video from a GoPro.
While not meant for the track, DailyRoads Voyager for Android is something I have used for short AutoX runs. It will allow you to embed GPS data in the video so speed will show up in a corner as digits. It will maintain a temp video folder with a size limitation you can adjust by deleting older video to make room as it records new files. You can permanently keep video by touching the screen while it is recording. I don't drive anywhere without it running, actually. It will help if I am ever in an accident with either car, I figure. I don't think a cop would believe a young guy driving a Boss was not the cause of an accident