NOVELTY - Improving (M1) carbon conversion efficiency of fuel production from carbonaceous feed material, involves: (A) providing a gaseous product stream which is formed from fossil-fuel-based hydrocarbon feed and which comprises hydrogen (H2) and carbon monoxide (CO) at a mole ratio of at least 2:1; and (B) adding the gaseous product stream provided in step (A) to syngas stream derived from carbonaceous feed material and containing H2 and CO to form a mixed syngas stream having H2:CO mole ratio higher than that of the syngas stream derived from carbonaceous feed material.    USE - For improving carbon conversion efficiency of fuel production from carbonaceous feed material useful for production of fuel (claimed) such as ethanol.    ADVANTAGE - The methods provide improvements in the efficiency of production of liquid fuels from the biomass as well as coal and petroleum coke. The methods use fossil fuels to increase the beneficial impact of liquid hydrocarbon fuels derived from biomass or other carbonaceous feed material on carbon dioxide emissions derived from use of fossil-based liquid hydrocarbon fuels. The methods improve carbon conversion efficiency of fuel production from carbonaceous feed material.    DETAILED DESCRIPTION - A method (M1) for improving the carbon conversion efficiency of fuel production from carbonaceous feed material, involves: (A) providing a gaseous product stream which is formed from fossil-fuel-based hydrocarbon feed and which comprises hydrogen (H2) and carbon monoxide (CO) at a mole ratio of H2:CO of at least 2:1; and (B) adding the gaseous product stream provided in step (A) to a syngas stream derived from carbonaceous feed material and containing hydrogen and CO, in a sufficient amount to form a mixed syngas stream having H2:CO mole ratio higher than that of the syngas stream derived from carbonaceous feed material. An INDEPENDENT CLAIM is included for a method (M2) involving: (A2) deriving a syngas stream from carbonaceous feed material, where the syngas stream contains hydrogen and CO; and (B2) adding to the syngas stream derived in step (A2) a gaseous product stream comprising hydrogen and carbon monoxide, where the mole ratio of H2:CO of the gaseous product stream is at least 2:1, and where the gaseous product stream is formed from fossil-fuel-based hydrocarbon feed, where adding the gaseous product stream to the syngas stream forms a mixed syngas stream having a H2:CO mole ratio higher than that of the syngas stream derived from carbonaceous feed material.