sounds AWESOME!!!!! You could also integrate a gear indicator! In the Netherlands there was a guy who was working on a project of adding a tablet as dashboard to his bike. unfortunately the bike got stolen and the project came to a hold.
If I'm not mistaken he also used an Arduino. If you could make this you can sell this big time! Not only can you display the Speed and RPM, but also temp, gear indicator, navigation (how cool is that) and he was even thinking about a rear viewing camera so he could eliminate the mirrors!
Keep up the good work!