A tunable diode laser absorption spectrometry system for fuel gas measurement, comprising: an off-axis integrated cavity output spectroscopy (ICOS) instrument comprising a first tunable laser diode with a first tunable wavelength range providing laser light coupled off-axis into an optical cavity , a second tunable laser diode with a second tunable wavelength range providing laser light coupled off-axis into the optical cavity , a gas inlet and a gas outlet arranged to flow a fuel gas through the optical cavity , and an optical sensor arranged to measure intensity of laser light exiting the optical cavity , wherein the first and second tunable laser diodes are tuned over their respective wavelength ranges so that the sensor measurement provides a wavelength-dependent optical absorption spectrum; wherein the first laser diode is tunable over a spectral range of at least 20 GHz, and wherein the second laser diode is tunable over a spectral range of 60-80 GHz; and a computer processor to receive the sensor measurement from the optical sensor , access a database of basis spectra for fuel gas components, process the absorption spectrum with a chemometric fitting routine to determine concentrations of selected fuel gas components, and calculate at least a heating value (F) and an additional value (X) selected from relative density, compressibility, theoretical hydrocarbon liquid content, and Wobbe index from the determined fuel gas component concentrations on the basis of determined concentrations for methane (C CH4 ) and ethane (C C2H6 ) and the determined concentration (C BB ) of an offset basis spectrum representing higher hydrocarbons such that: X = C CH4 · X CH4 + C C2H6 · X C2H6 + C BB · E, where X CH4 and X C2H6 are respective coefficients for methane and ethane, and E is an empirical factor for a composite relative density, compressibility, theoretical hydrocarbon liquid content, or Wobbe index of all expected higher hydrocarbons in the fuel gas mixture. an off-axis integrated cavity output spectroscopy (ICOS) instrument comprising a first tunable laser diode with a first tunable wavelength range providing laser light coupled off-axis into an optical cavity , a second tunable laser diode with a second tunable wavelength range providing laser light coupled off-axis into the optical cavity , a gas inlet and a gas outlet arranged to flow a fuel gas through the optical cavity , and an optical sensor arranged to measure intensity of laser light exiting the optical cavity , wherein the first and second tunable laser diodes are tuned over their respective wavelength ranges so that the sensor measurement provides a wavelength-dependent optical absorption spectrum; wherein the first laser diode is tunable over a spectral range of at least 20 GHz, and wherein the second laser diode is tunable over a spectral range of 60-80 GHz; and a computer processor to receive the sensor measurement from the optical sensor , access a database of basis spectra for fuel gas components, process the absorption spectrum with a chemometric fitting routine to determine concentrations of selected fuel gas components, and calculate at least a heating value (F) and an additional value (X) selected from relative density, compressibility, theoretical hydrocarbon liquid content, and Wobbe index from the determined fuel gas component concentrations on the basis of determined concentrations for methane (C CH4 ) and ethane (C C2H6 ) and the determined concentration (C BB ) of an offset basis spectrum representing higher hydrocarbons such that: X = C CH4 · X CH4 + C C2H6 · X C2H6 + C BB · E, where X CH4 and X C2H6 are respective coefficients for methane and ethane, and E is an empirical factor for a composite relative density, compressibility, theoretical hydrocarbon liquid content, or Wobbe index of all expected higher hydrocarbons in the fuel gas mixture. <EOS>