Methods and systems for use in determining one or more performance characteristics of a mechanical system that includes a combustion engine. One or more values of a physical property of fuel are associated with a performance characteristic (e.g., an operating characteristic and/or a maintenance characteristic) of a mechanical system to create a fuel-performance relationship. An actual value of the physical property of a fuel to be burned by the combustion engine is determined using one or more sensors. A predicted value of the performance characteristic is determined based on the actual value of the physical property and the fuel-performance relationship.