A semiempirical correlation was developed for predicting the laminar flame speed of H2/CO/air flames with N2 and CO2 dilution without solving the detailed governing equations, detailed chemical kinetics, and molecular mass transport. The correlation, derived through an asymptotic analysis, comprised a theoretically based expression with a series of experimentally fitted parameters. New experimental measurements were conducted using the Bunsen flame method over broad ranges of unburned mixture temperature (300-600 K), H2 ratio in the H2/CO blends (0.25-0.75), CO2 or N2 dilution ratio (0-0.67), and equivalence ratio (0.8-4.0) to comprehensively validate the performance of the proposed correlation. Detailed numerical simulation and sensitivity analysis using CHEMKIN-Pro were also carried out to gain an insightful understanding of the experimental observations. Results showed that the proposed correlation was able to satisfactorily predict the laminar flame speed, with error < 15%, of a wide range of H2/CO/N2/CO2/air mixtures covering H2 ratios in the H2/CO blends from 0.25 to 1.0, N2 and CO2 dilution ratios from 0 to 0.25, equivalence ratios from 0.8 to 4.0, unburned gas temperatures from 300 to 500 K, and pressures from 1 to 2 atm. When the unburned gas temperature was raised to >500 K or the mixture was highly diluted (dilution ratio up to 0.67), the proposed correlation could be conditionally applied since the prediction error would reach 15-30%. The sensitivity analysis demonstrated that the relatively poor prediction of the proposed correlation under high unburned temperature or high dilution conditions was mainly due to the variation of the chemical kinetics. The proposed correlation generally provided a much simpler and more convenient approach to estimating the laminar flame speed of H2/CO/N2/CO2/air flames with an acceptable accuracy, especially suitable for applications in the engineering computations.