Abstract: If a throttle valve is fully-closed, it is determined whether or not a transmission is to be downshifted, by determining whether or not an acceleration of the vehicle is equal to or greater than a downshift criterion value. The downshift criterion value is set as a value that decreases as the vehicle speed deviation, which is a difference between a vehicle speed and a target vehicle speed, increases. By using a downshift criterion value, an increase in the vehicle speed deviation resulting from a quick downshift on a steep downhill can be restrained. On a gentle downhill, a downshift is not performed even if the vehicle speed deviation assumes a significant value. Thus, it becomes possible to inhibit the vehicle speed from becoming lower than the target vehicle speed during a short period after downshift. Accordingly, an increase in fuel consumption and deterioration of noise of the engine can be restrained.

Abstract: If a throttle valve is fully-closed, it is determined whether or not a transmission is to be downshifted, by determining whether or not an acceleration of the vehicle is equal to or greater than a downshift criterion value. The downshift criterion value is set as a value that decreases as the vehicle speed deviation, which is a difference between a vehicle speed and a target vehicle speed, increases. By using the downshift criterion value, an increase in the vehicle speed deviation resulting from a quick downshift on a steep downhill can be restrained. On a gentle downhill, a downshift is not performed even if the vehicle speed deviation assumes a significant value. Thus, it becomes possible to inhibit the vehicle speed from becoming lower than the target vehicle speed during a short period after downshift. Accordingly, an increase in fuel consumption and deterioration of noise of the engine can be restrained.