If you traveled via vehicle, you should not enter the car expenses in that spot. Enter the lowest return airfare for the destination. Enter the actual expense amount from traveling via vehicle in the spot marked "Travel Expenses".

CRA requires that information as the calculation for the allowable amount has to be the lesser of these three amounts:

• the taxable travel benefits you received from your employer for the trip;
• the total travel expenses paid for the trip; or
• the cost of the lowest return airfare available at the time of the trip between the airport closest to your residence and the nearest designated city.

http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns248-260/255/clc-eng.html