Quantisation is a potential factor but the graph does not show its effects

Pardon me? The blue graph is obviously a result of sampling and quantization of the red graph. If there was no quantization but only sampling going on there would be exclusively vertical blue lines with precise values instead of quantized values and no horizontal blue lines because no data between samples. To be precise, the blue graph does not even show the precise values of the samples but only the results of the quantization of those. Exact sample values are only indirectly in this graph - they are where red graph and blue vertical lines meet.

However - I was primarily referring to OP´s idea that digital speedometers would be more precise than analog speedometers. If you look at the graph you will see that the analog speedometer always knows and thus displays the exact speed of the car in any moment (plus a small inevitable speedometer system delay). The digital speedometer on the other hand most of the time only knows the quantized value of the last taken sample - except in the exact moments when the samples are taken. Considering the quality (resolution and speed) of nowadays digital technology I assume this is not a factor to consider when designing speedometers though.