The fuel economy of the KTM 390 Adventure can vary depending on various factors such as riding conditions, terrain, rider weight, and riding style. However, as per the manufacturer's specifications and user reports, the KTM 390 Adventure typically offers a fuel economy of around 70-80 miles per gallon (mpg) or approximately 30-34 kilometers per liter (km/l) under highway cruising conditions.
It's important to note that fuel economy can be influenced by many variables, including speed, wind resistance, elevation changes, and individual riding habits. Therefore, these figures are estimates and actual fuel economy may vary. If you are looking for more precise information, it's recommended to refer to user reviews or consult with KTM dealers or owners who have firsthand experience with the KTM 390 Adventure at highway speeds.