To display the car speed I'd rather choose the most passive, simple technology while having a large digit size. It is out of question to use something that emits light.

E-paper could do the job, but large displays are pretty expensive, and I'm not sure if they can handle the rough condition of a car parked in sunlight or a freezing winter.

Electro-mechanic Flip display would be a good choice if only the big 7 segments units were not so expensive. Besides, I'm not sure the continuous flipping would be good for the display and the driver ears.

Fortunately we can still find good old fashion 7 segments LCD in large size. I yet use some of these panel on the BIG_CLOCK project. The biggest apparently are made by Lumex, the LCDS101D40TR. The character size is 10x7 cm.

Only two digit is relevant in my opinion. At a speed faster than 100Km/h, digits should be unreadable due to the safety distance. So we have to drive only 14 segments. The PIC16F19156 can drive them directly, without multiplexing.

The code simply take the received byte on the serial port and display its decimal value from 0 to 99.