Cruise speed and fuel adjustments are available for all ForeFlight Performance Profiles so that you can adapt our model to your aircraft if the two differ consistently. Before making any changes, fly at least 10 flights while closely tracking your actual fuel burn and flight times. After each flight, compare your actual values to the time and fuel burn predicted by ForeFlight. If the actual values differ from the predicted values in a consistent and predictable fashion (such as actual values always burns ~3% more fuel than predicted), then use the model adjustment sliders for that cruise profile to adjust the ForeFlight model closer to yours.
To make an adjustment, go to MORE > AIRCRAFT and select the aircraft to modify. Under the Performance section, tap on PERFORMANCE PROFILES and then select a Profile to modify. Move either the CRUISE SPEED or CRUISE FUEL FLOW sliders as determined from your tests.