As your wife is having consulting income, this income will be treated under the head "Income from Business & Profession". All reasonable expenses incurred in connection with the earning of such income, including depreciation, interest, vehicle maintenance etc. can be claimed as expenditure against such business / professional income. If there is loss from such business/profession, it can also be set off against the income from house property and income from other sources as per the provisions of the Act. The only relevant test is that the expenses being claimed should be reasonable and attributable to the business/profession.