Jakg- CGI always looks a bit too perfect. Not only is the shading is always easily definable from real video, you also don't get the sharp edges around objects that real DV tape gives you, and you tend not to get interlacing etc etc. Have you ever seen a CGI video you could look at and say "Yeah, that's real"?
As for the speedo itself being edited, again that's incredibly difficult to do because not only do you have to match the camera shake, you also have to get the film grain on top of that, not to mention if the camera goes out of focus for a few frames, that also has to be matched in the CGI speedo.
There are so many things that separate CGI from real video and if someone had the ability to create something that convicing, they wouldn't be posting it on Youtube- they'd be working for ILM or something.
God, I put way too much thought into that. Bottom line, the guy's an idiot.