In the process of formulating management decisions, the government and enterprise organizations need to forecast the future market trends and economic environment. High-accuracy forecasts will help governments and organizations to make the most appropriate decisions and avoid the organization’s difficulties caused by improper decision-making. Due to the geographical environment, Taiwan had a large number of motor vehicle registrations. In this study, the number of registrations of small passenger cars in Taiwan is the subject in this research. The specific objectives are as follows: (1)establishing a GM (1,1) grey prediction model for the number of registered passenger cars through the historical registrations number over years; (2)exploring the degree of correlation between external factors affecting the number of registrations of passenger cars by the aid of the grey correlation analysis; (3)adding predictions through complex regression analysis and evaluating models for accuracy comparison. The results of this study show that when the ranking of the correlation factors is considered to be significant, the highest accuracy rate for predicting the car registration number can be obtained when seven factors are included, such as the number of households, net household savings, average national income per capita, average per capita consumption expenditure, oil price, economic growth rate, and the ratio of couples greater than fifteen of age.