You'll find the speedo will read about 4mph than your actual speed. I've tested this against speed signs and my sat nav and the result is consistently the same - 3 to 4 mph higher on the speedo It's not just the mazda - it's the same difference on my golf and jaguar too. ...
It is generally the same for EVERY car, no matter what make. Some say it is purposely done to keep you at a lower speed than is actually indicated, with a view to trying to keep you under the imposed speed limit whatever that may be. This of course relies upon your own conscience.
Others will say it's just a function of the mechanical connection (Speedo cable) between gearbox and speedo head.
Whatever the reasoning, as you say this can be compared to a GPS speed display on your Sat Nav.
I've never tried with an electronic display. It would be interesting to see if that too is "purposely" inaccurate.
However as has been remarked several times before, any automatic method of registering fuel consumption, as in an OCB, is going to be notoriously inaccurate. There is no real substitute for brim to brim calculations.