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 fuel injection amount calculating means for calculating a fuel injection amount per predetermined unit from the accelerator opening degree and the measured rotational speed, based on 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 the fuel injection amount; and a mileage evaluating section for effecting a mileage evaluation based on the fuel injection amount calculated by the fuel injection amount calculating means.