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Variations in fuel composition can cause non-optimal natural gas engine performance and may affect engine durability. The sound speed in natural gas can be used as a measure of fuel quality, since it changes with the molecular weight of the gas. Sound speeds measured for twelve gas compositions matched theory. Based on over 6700 gas compositions, the sound speed correlates well with percent methane, percent non-methane hydrocarbons, density, and methane number. Moderate correlation results for the air-fuel ratio, hydrogen-to-carbon ratio, Wobbe index, and lower flammability limit. A sound speed sensor could provide input for engine control based on the quality of the natural gas.