Heres what I gathered on a quick test. Using my iPhone 4S with the garmin app i watched vehicle speed compared to what the gps reported.
From previous times I know my iPhone reads the same as my stand alone garmin 850 (IIRC on the model number, it was top of the line when I bought it a few years back).
Anywho, the iPhone matched the digital display for my MPH readout.
As for the analog gauge here's my opinion. Parallax!
Basically, I can make the analog speedo read quite different depending how I look at it and from what angle.
I'm sure it's accurate though.
Thoughts and opinions? But I gather both read correctly.
Sent from my iPhone 4S