I am in the process of buying another car now, and one of the resources I used most of Edmunds.com's "True Cost to Own" feature. (As an example, look at this TCO report of my Camry.)

The True Cost to Own in Edmunds' definition includes the following seven parts:

Depreciation: This is the amount by which the value of a vehicle declines from its purchase price.

Insurance: This is the estimated average annual insurance premium being charged by insurers in your state. The premium has been determined based on industry-wide, state-by-state data from the National Association of Insurance Commissioners (NAIC)*

Financing: This is the interest expense on a loan in the amount of the True Market Value® purchase price (including typical equipment) plus the destination charge (for new cars) and the base sales tax and initial fees for your state, assuming a 10% down payment and a loan term of 60 months. The interest rate used is the prevailing rate that banks and other direct automotive lenders are charging consumers.

Taxes & Fees: Sometimes referred to as DMV fees, this consists of the base sales taxes, license and registration fees in your state, and gas guzzler tax if applicable.

Fuel: This expense is based on EPA mileage figures, assuming consumption consists of two-thirds highway and one-third city driving, and that the vehicle has an automatic transmission unless automatic is not available.

Maintenance: This is the estimated expense of two types of maintenance: scheduled and unscheduled. Scheduled maintenance is the performance of factory-recommended items at periodic mileage and/or calendar intervals. Unscheduled maintenance includes wheel alignment and the replacement of items such as the battery, brakes, headlamps, hoses, exhaust system parts, taillight/turn signal bulbs, tires and wiper blades/inserts.

Repairs: This is the estimated expense for repairs not covered by the vehicle manufacturer's warranties over the five years from the date of purchase, assuming an average of 15,000 miles are driven annually. This expense is based on the cost of a typical "zero deductible" extended warranty for the vehicle, minus the estimated amount of that cost that consists of the warranty provider's overhead and profit.

I found the above classification very useful, and in the case of my Camry, the depreciation, taxes and fees, and fuel cost are very much in line with my actual expenses. I paid cash for my car, so I don't have direct financing expenses, but I do have an opportunity cost of having my cash locked in the car. The TCO report overestimated my insurance, maintenance and repairs cost -- that can be because I comparison shopped my insurance, and my car is relatively of better condition compared to its peers.

When it comes to buying another car, I found the TCO report very useful in pointing out the depreciation potential of different makes, and some surprises in expected maintenance and repair cost. The calculated average cost per mile is also very indicative of real car ownership cost.