That depends on your calibration, technique, and the sport. For the best calibration you need to use a treadmill, maintain a contstant speed, and ideally (if you are comfortable with it) run a 10-minute mile pace (6.0mph). This exactly matches the studies that we have based the algorithms on. In regards to the sports: Running is typically within 1MPH and biking within 2mph of the speed that you would be going if you were there in person. This margin may increase at low and very high speeds due to a number of factors in physiology. Some of these factors include a person's ability to push themself at threshold and also their efficiency/economy. For sports that require precise technique (swimming, climbing, rowing, etc.) the speed prediction is more directional in nature. Lastly, heart rate responds over time and so FitTrip will average out interval-type sprints where intensity is quickly changed.