When I last visited China, the vast majority of people had Samsung phones.

I saw a particular preference for large screen devices.

IMO it will be difficult to get the Chinese to pay the price of a Galaxy Note for a device with a 4 inch screen.

The speedometer will be configured to respond to a specific CAN message.

For example, in J1939 the vehicle speed is contained in a message formatted as follows:

Header ------------- Bytes 1-8
18FEF100 __ ll hh __ __ __ __ __

The speedo filters messages which have the header __FEF1__ and convert bytes 2 and 3 into speed, then displays it.
The hackers devices merely blasts the bus with erroneous messages.