The composition of end-use natural gas is known to vary significantly around the United States. Of primary interest for this analysis was the impact of natural gas fuel composition on fuel metering and engine operational characteristics. A fuel metering model was developed to analyze the impact of fuel composition on carbureted, fuel-injected (premixed), and direct-injected engine configurations. The change in physical properties of the fuel was found to have a profound effect on fuel metering characteristics. Fuel composition was found to affect different fuel metering configurations differently, but these variations were minor compared to the fuel property effects. Equivalence ratio variations of 12 percent lean and 6 percent rich were calculated from a median gas composition. Wobbe Index was identified as a key parameter for estimating the change in metered equivalence ratio between two fuels. Fuel composition also affects the lean-flammability-limit of the mixture which, when combined with fuel metering variations, can cause a lean-burn engine to misfire. Fuel temperature variations affect fuel metering and must also be considered. Results indicate that closed-loop mixture control is essential for stoichiometric engines and very beneficial for lean-burn engines. A closed-loop feedback system based on exhaust oxygen concentration is the preferred approach.