(1) A conventional index adopted for giving the number of drive wheels of a motor vehicle. The first number corresponds to the overall number of wheels and the second, to the number of drive wheels (for example, the ZIL-130 automobile is 4 X 2; the GAZ-66, a 4 X 4; and the ZIL-131, a 6 X 6).
(2) A conventional numerical (or letter) index for the running gear of a locomotive, also termed the axle formula, which indicates the number and arrangement of wheel axles. The first number of the wheel formula is the number of axles of the leading truck, the second, the number of driving axles, and the third, the number of axles of the trailing truck. Modern locomotives as a rule do not have leading and trailing axles, and the driving axles are joined into four-wheel or six-wheel trucks that are described by the wheel formulas 2–2 or 3–3 (in the Soviet Union) or B-B or C-C (abroad). For electric locomotives, a subscript is added to the numbers indicating that the driving axles have traction motors (for example, the VL-10 electric locomotive has the wheel formula 20-20-20-20).