I like the concept. In theory it could run off GPS signal for speed so tire size etc isn't a factor.
They can also be fitted with wifi so MPG tracking could be sent to a server (such as on here or other MPG tracking sites).
I wonder if there are any ultra wide LCD screens that would be about the right size to replace a stock analog gauge cluster. Could layer it with a touch sensitive layer for changing settings and such (while stopped only).
I personally would rather have like a bar graph or similar to indicate speed with the digital readout near it. Kind of like the rpm gauge in the bottom right of this stock cluster from the early 90's. The biggest thing is the concept, could have more info displayed in an easier to understand format. With GPS it could record hills and such too.