The app uses your location in order to track your mileage, similarly to a navigation app like Waze or a rideshare app like Uber. Using location can cause a strain on battery, since these apps need to find your location at least once per minute in order to give you an accurate reading.
One way to reduce strain on your phone's battery would be to close any apps that you aren't actively using. This will give your phone's battery a break!
Making improvements to how the app affects battery life is always top-of-mind for us, and we make tweaks to improve battery life in every new version of the app that we release.