The best Upside promo code is NINJA30. It provides new users with a 30¢/gal bonus on their first fill-up using the app. This bonus is on top of the standard 1¢/gal to 25¢/gal offered to all users, so the total cash back on your first fill-up could range from 31¢/gal to 55¢/gal depending on the gas station you're visiting.