Whoa that's pretty neat. Do you have a physical prototype yet like a 3D printed case around it, battery, etc? I know you mentioned GPS but wondering if you would consider a hardware input like an rpm reader on the tires.
I will sell them together with a Bluetooth wheel/cadence sensor once I figure out Bluetooth.
I'd be curious on design choices like waterproofing, battery life, things like that. Also I think you mentioned your site is getting worked on but pictures to clarify what it is about would help sell it.
The web site was put up before I started the firmware work, so obviously no pictures :) They will come once the project becomes more useful.