I've thought about that too (well, not the advertisement bit). FYI, MPG on the rear deck could theoretically be done to a mpguino for about an extra $10 in parts and some fancy finger work.
Basically you would buy a bunch of LEDS and build an oversided LED number display (think black cardboard with a bunch of holes in it and hotglue). Then get an additional atmega168 and program it with your duino board to accept a number over the serial port and display it on the jumbo LED display.
Note: there are enough pins on a duino/atm168 to support up to an 8 digit numeric LED display without extra chips, but you would need 64 transistors to control the segments. So probably worth it to get a MAX 7219 like in the arduino playground example:
http://www.arduino.cc/playground/Main/LedControl