Engine oil and automatic transmission fluid viscosities are major factors in assuring good starting and running performance in cold weather. To determine the contributions of the engine and transmission to the cranking and running effort, instantaneous torque and power, obtained with an instrumented engine-transmission apparatus, were determined for five engine oils ranging in viscosity from 4 to 184 poise (SAE 5W to SAE 20W) and for four transmission fluids ranging in viscosity from 3200 to 83,000 cp at -20 F. Specific engine and transmission cranking variables - engine friction, compression and expansion, engine rotational inertia, and transmission friction and rotational inertia - were analyzed in detail.The engine required most of the cranking effort, which increased with increasing engine oil viscosity. Increasing engine oil viscosity increased engine friction torque but decreased engine friction power because of decreased cranking speed. Net cylinder torque input to the cylinder air, a result of compressing and expanding air in the cylinder, decreased with decreasing cranking speed. Both engine and transmission inertia could be neglected for most cranking conditions.A thermodynamic analysis of the cranking process revealed that cranking speeds should be high for good starting because heat losses from the cylinder gases are low and cylinder gas pressures and temperatures are high.A crankability factor, defined as the ratio of instantaneous engine friction torque to cranking speed, indicated that engine crankability deteriorated drastically as engine oil viscosity increased above about 30 poise and starter motor stall was being approached.Results obtained with the engine-transmission apparatus were corroborated by results obtained in car tests. In general, starting and running improved as the engine oil and transmission fluid viscosity were lowered. Transmission performance was unsatisfactory with fluid viscosities greater than 30, 000 cp, as a result of inadequate fluid flow. Because of engine stall, a transmission fluid viscosity less than 3200 cp is desirable for low temperature running.