OMG - I’ve found how to change the arrow to a car in google maps - it looks great on apple car play

Try it !

Open Google Maps on your device.
Press the blue Directions icon.
Set your destination and route.
Tap Start.
Once your navigation has begun, tap the vehicle icon (represented as a blue arrow)
By swiping from left to right and then tapping on the desired car, choose your new vehicle icon.