I've had very good luck with the Cat-eye Micro wireless computers, my wife still use them. They work in all conditions(don't know about -20), but rain, fog, snow and heat hasn't killed them.
Now I use a Cat-eye gps unit my son got me a few years ago. I works well, hold a charge a long time. The beauty of it is that I can move it from bike to bike with out having to change any settings, you just have to buy extra mounts for it. The only real problem is that it sometimes loses the GPS signal under heavy leaf cover. The ride details can be loaded to something such as Strava so you can track you mileage etc..