1. A condition evaluation system for an engine-driven traveling vehicle, comprising: a rotational speed detecting section for detecting an engine rotational speed of the engine-driven traveling vehicle as a measured rotational speed; an accelerator opening degree detecting section for detecting an accelerator opening degree; a storing section for storing, as a correlation group, a correlation prepared for each accelerator opening degree between an engine rotational speed drop amount from a reference rotational speed which is the engine rotational speed under a zero load condition of the engine and a change in fuel injection amount; a fuel injection amount calculating means for selecting, from the correlation group, the correlation associated with the accelerator opening degree detected by the accelerator opening degree detecting section, and applying, to the selected correlation, the measured rotational speed detected by the rotational speed detecting section to calculate a fuel injection amount per predetermined unit; and a mileage evaluating section for effecting a mileage evaluation based on the fuel injection amount calculated by the fuel injection amount calculating means.