If you own your own business chances are that you use your car for your business. You pick up inventory, or supplies, perhaps you visit clients, or hopefully go to the bank to do deposits. If you use your car for anything like the above examples you can expense a portion of your use to your business. That said, there is (of course) some rules that apply in order to stay clear and clean with the Canada Revenue Agency.
The first rule and recommendation is that you log your travel. If you want to expense it you must quantify it, and have support for your expenses. Keep a book in the car and log the date, mileage and reason for each trip. This will go a long way in determining the cost related to your car. Also if you ever get audited you can prove the business use of your vehicle. Keep all your receipts for gas, maintenance, leases etc as they will be needed to calculate costs, and the percentage that can get allocated to your business.
As with everything for a small business, things need to be kept in proportion with the business itself. For example, if you are a hair dresser with a storefront, your mileage will be minimal, as you will more often than not be in your store working, however if you are a real estate agent, your car is your office and your travel will be extreme. Keep it in line with what you do and keep a log of travel and you will be able to take advantage of your vehicle expense.