[Istas]

macchesney: free lite + paid pro sounds like a good compromise; it would probably be used much more widely, and would let people see if the GPS units in their phone are accurate enough for it to be useful. (Incidentally, I would advise putting as prominently as possible somewhere obvious to see, "Test in clear skies and open areas for maximum precision; cloudiness and tall trees and buildings can ruin results."

In fact maybe you could make the pro version be "pay what you want" (with a minimum donation to eclipse the fees charged by payment methods); that way if someone's really appreciative they can show you.

If you're using local weather data, it'd probably be good to state how far away the station is; here in Montana at least it can be an hour's drive away, and change by 15 miles on a daily basis (depending on whether a personal weather station is actually functioning).

I believe correcting for gradient at highway speed would require having a fair idea of what the rolling resistance is (and that would also help get better precision on the cD calculation, obviously). You could have a low-speed rolling-resistance calculation too.

Other ideas: options to average multiple runs, as well as designate whether a run was heading east or west (or north-south, or headwind/tailwind, etc), and catagorizing saved runs under different settings (like snow tires, summer gas, cargo, different mods, etc).

...Reading up, I realize that's a lot of extra stuff. Don't mean to blather away, just throwing ideas out there.