A numerical study is conducted to investigate the effect of changing engine oil and automatic transmission fluid (ATF) temperatures on the fuel economy during warm-up period. The study also evaluates several fuel economy improving devices that reduce the warm-up period by utilizing recycled exhaust heat or an electric heater. A computer simulation model has been developed using a multi-domain 1-D commercial software and calibrated using test data from a passenger vehicle equipped with a 2.4 / 4-cylinder engine and a 6-speed automatic transmission. The model consists of sub-models for driver, vehicle, engine, automatic transmission, cooling system, engine oil circuit, ATF circuit, and electrical system.The model has demonstrated sufficient sensitivity to the changing engine oil and ATF temperatures during the cold start portion of the Federal Test Procedure (FTP) driving cycle that is used for the fuel economy evaluation. The results from the study indicate that the potential fuel economy improvement during the driving cycle is 7.3 % at 24°C ambient temperature, and 20.1 % at −6.7°C. An electric ATF heater and two heat exchangers that recycle exhaust gas to heat ATF or engine oil have been evaluated in terms of fuel economy improvement. The study has discovered that the exhaust ATF heater has the greatest impact on the fuel economy during the warm-up period. According to the simulation results, the improvements are 2.1 % at 24°C ambient temperature, and 7.2 % at −6.7°C.