Capital gains computation must use historic cost for motor cars despite depreciation cost limits, affecting gain or loss calculation. For capital gains on sale of a motor car, the historic and true cost of acquisition is to be taken, notwithstanding the proviso that limits 'actual cost' for depreciation purposes. The definition of 'actual cost' in the income provisions is confined to sections relating to business income and depreciation; it should not be extended to capital gains computation. Written down value and proportionate adjustments apply for depreciation and related business profit adjustments, but the full purchase cost governs capital gains calculation.