I checked my Garmin LM2455, and though there are multiple different dashboards that show your GPS based speed, none of them will fill the screen. For what you're looking for, a speedometer phone app is probably your best bet. Rather than investing the money in a standalone electronic device that may or may not exist, just look for a mount that will position your smart phone where you want it, and put the speedometer app on it. I have a Droid Turbo, and the app I used to use filled the whole screen, plus gave you other options like a tripmeter.
"No matter where you go, there you are."