It is widely understood that cold ambient temperatures negatively impact vehicle system efficiency. This is due to a combination of factors: increased friction (engine oil, transmission, and driveline viscous effects), cold start enrichment, heat transfer, and air density variations. Although the science of quantifying steady-state vehicle component efficiency is mature, transient component efficiencies over dynamic ambient real-world conditions is less understood and quantified.This work characterizes wheel assembly efficiencies of a conventional and electric vehicle over a wide range of ambient conditions. For this work, the wheel assembly is defined as the tire side axle spline, spline housing, bearings, brakes, and tires. Dynamometer testing over hot and cold ambient temperatures was conducted with a conventional and electric vehicle instrumented to determine the output energy losses of the wheel assembly in proportion to the input energy of the half-shafts. Additionally, response surface methodology (RSM) techniques were applied to the conventional vehicle serving as predictive models of the wheel assembly efficiency as a function of its thermal state. For the conventional vehicle, data showed that under -17°C ambient conditions, nearly 40% of the wheel assembly efficiency is lost over an urban drive cycle. For the urban cycle driven at +35°C, this loss reduces to less than 10%. For standard +20°C ambient conditions, the efficiency of a first urban cycle for a conventional vehicle is on the order of 78%, increasing to nearly 85% by the third cycle. For a battery electric vehicle, the first urban cycle at -7°C losses are on the order of 40%. At +35°C, these losses are reduced to approximately 5%. Efforts to reduce such significant losses could positively impact vehicle system efficiency.