I use Strava on a phone for hikes and after 2 hours, the battery is down to 50%. My Garmin 500 battery life is a lot better. How big is the difference in speed and distance and for how many miles? You can probably google and find people who know what the algorithms are these devices use and why there are differences. If you have a lot of hills, the gps might be shorter because you get the projection of the hill onto a "flat" earth rather than the true distance.