I suggested developing a phone to use as a "head unit" on these forums in February 2008. Everyone here thought it was a crazy idea. No phone even had gps yet. It wasn't crazy at all, as your post shows. A Garmin is nearly the price of a gps-enabled smartphone. But battery life is the problem.

My suggestion is Android phones like a Galaxy S5, where you can use a massive battery pack built into the back of the phone. But then you'll need a special case to fit that extra battery size and I'm not sure they're out there. Another possibility: with the S5, you can buy extra batteries for $10 a pop. Pack three of them and swap out at a convenience store. Not sure if the software will keep your file up and running, though, or whether you'd need to restart a new ride each switchover.

My 6-year-old pre-Strava thread.

http://www.bikeforums.net/road-cycli...head-unit.html