I like the way you used the LCD to display large characters.
Hey the speedometer works on the principle of Doppler's effect? Is it? I've read it somewhere.
Thank you, but I can't take credit for the large characters. I was inspired by this thread:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1245352653
I'm not sure if Doppler's principle would apply to this particular design. Maybe more so if I were measuring my speed with a radar. 
For power I'm running off the vehicle's battery, but I did take some protective measures. There's a dedicated line that connects very close to the battery itself, which from what I understand makes a pretty significant filter. I plan to use a key-on powered relay in the future, but for now I just plug in the PCB when I'm going to drive somewhere. I also have a simple overvoltage/overcurrent protection circuit that I constructed in the power cable:
So far it's put up with almost daily use for about 9 months with no trouble. 