1. A vehicle planning support system for supporting the planning of new vehicles by building vehicle models, comprising: a database for storing vehicle data for building vehicle models, and a computing device utilizing the vehicle data stored in the database to build planned vehicle models for the vehicle being planned, wherein performance evaluation values for quantifying performance evaluations of the vehicle with respect to each of a predetermined evaluation items are included in vehicle data stored in the database for a plurality of existing vehicles, timing-related correction data indicating trends in time period-related changes in performance evaluation values based on performance evaluation values for past and present existing vehicles for each evaluation item is stored in the database, and the computing device has a performance evaluation values distribution defining section for defining a distribution of performance evaluation values of at least a section of comparison subject existing vehicles among the plurality of existing vehicles with respect to each evaluation item based on the data of the performance evaluation values for quantifying performance evaluations for a plurality of existing vehicles stored in the database, an initial target value setting section for setting initial performance target values of the planned vehicle with respect to each evaluation item based on the performance evaluation values distribution which is defined by the performance evaluation values distribution defining section, for at least a section of comparison subject existing vehicles among the existing vehicles, a final target value setting section for setting final performance target values of the planned vehicle with respect to each evaluation item by correcting the initial performance target values to match the planned vehicle selling period, based on the timing-related correction data, a performance calculation section for computing performance evaluation values of the planned vehicle with respect to each evaluation item, and a target degree-of-completion calculation section for obtaining a target degree-of-completion by comparing the final performance target values and performance evaluation values of the planned vehicle with respect to each evaluation item.