PURPOSE: To provide a speed calculation indicator for correcting a vehicle speed indicated after making its correction on the basis of a change in the tire diameter.

CONSTITUTION: When a pedalling amount of an accelerator pedal 10 detected by an accelerator sensor 11 is maximized, a controller 7 detects actual electric current I flowing in a travelling motor 1 through a current sensor 5, obtains the desired rotational frequency No of the travelling motor 1 corresponding to the actual current I from a memory section 7a and compares the obtained desired rotational frequency No with the actual rotational frequency N of the travelling motor 1. When the desired rotational frequency No does not coincide with the actual rotational frequency N on the basis of the comparing result of the controller 7, the controller 7 judges a tire diameter of a vehicle to be changed and calculates a correction factor on the basis of an increasing and decreasing ratio of the actual rotational frequency N on the desired rotational frequency No. The controller 7 calculates the speed V of a vehicle on the basis of the actual rotational frequency N of the travelling motor 1 and the correction factor calculated by the controller 7 and indicates the calculated speed V on a display 8.