For all the angst over fluctuating petrol prices, fuel represents only a fraction of the cost of owning a car - and the biggest expense occurs even if your vehicle never leaves the driveway.

"A lot of people are surprised, if not a bit shocked, at the total cost of owning and operating a vehicle," says Michael Case, chief engineer of vehicles at the RACV.

"They mostly accept that they have to have the vehicle and ... generally equate fuel costs with the costs of owning a vehicle. But when you add it all up, it's much more than that."

The surprise comes from what Case calls the hidden costs of car ownership. Depreciation is the biggest component of those.

In a survey of 60 popular car models this year, the RACV found the fixed costs of car ownership ranged from about $84 to $118 a week for some of the smallest cars on the market. These included popular compact vehicles such as the Holden Barina and the top-scoring Hyundai Getz. Meanwhile, the weekly petrol costs for those cars ranged from $20.65 to $30.57. Among large four-wheel-drives, the gap was more stark. A Toyota LandCruiser, for example, costs its owner $295 a week, whether it's driven or not, and typically runs up a weekly petrol bill of $57.

The survey highlights the difference between fixed costs and running costs. (See racv.com.au and look under "my car" and then "advice and information" to find vehicle operating costs.)

Fixed costs include depreciation, loan interest, registration, insurance, licensing and membership of a roadside assistance group. Running costs includes fuel, tyres, maintenance and repairs.

The Victorian study used data collected in April and May, when petrol prices were roughly equivalent to what they are now.

"But while the [fuel price] varies, it's the relativity between these numbers that is important," Case says. "You can see enormous relative differences between these categories and even within some categories."

In every case, fixed costs far outweighed running costs, mainly because of the drain of depreciation - that is, the car's diminishing value as it gets older. The RACV study found depreciation for a LandCruiser accounted for about 45 per cent of the weekly costs of vehicle ownership.

"Depreciation is, by far, the greatest cost of buying and running a new vehicle," says Jack Haley, the manager of motoring research at the NRMA, which conducts similar research. (See mynrma.com.au and look under "motoring" for "car operating costs".)

"You don't see the depreciation ticking over every week," he says. "You only find out about it when you come to sell the vehicle."

Depreciation, he says, can be "particularly savage" in the first two or three years of a car's life, when at least half of its value evaporates.

The 2007 version of the NRMA study, which examined 555 models, found the impact of depreciation was lowest for small cars, where, for the first five years of a car's life, it typically accounts for 37 per cent of all costs.

Not surprisingly, given their higher sticker price, the impact is largest among big four-wheel-drives, where the loss during the same period accounts for 49.4 per cent of ownership costs.

In comparison, petrol consistently ranges between 17 and 19 per cent of total running costs.

The study assumed an average vehicle travels 15,000 kilometres a year.

Without question, different vehicles suit different families and safety features and environmental concerns play a role in selecting a car. Yet Haley advises people to use the data from such surveys to help them gauge the financial aspects of buying and operating a new car.

He stresses, though, that the information has implications for used cars, too. The biggest factor is the age of the car. Different cars depreciate at different rates.

"After two or three years, and certainly four or five years, most of the depreciation has already occurred," Haley says.

The importance of this is that buyers should then focus on other aspects.

"If they're buying a second-hand vehicle, which is much cheaper, then fuel becomes a greater part of the total running cost," Haley says.

Petrol prices have increased from when the NRMA survey was produced - unleaded petrol was quoted at $1.26 a litre in last year's study - but Haley says the net effect isn't enough to alter the significance of a car's fixed costs.

"Fuel is only a relatively minor part of the total costs of running the vehicle," Haley says. "And even with the increase in fuel costs over the last year, it's still a relatively small part.

"Everybody is sensitive to fuel costs - unless somebody else is paying for it - because they're always putting their hand in the pocket. Every time you refuel, you know how much it costs."

Another big expense highlighted in the NRMA study is "opportunity interest". This represents the money a person could have earned if the money lost through depreciation had been invested in a five-year bond. (The NRMA figures exclude the costs of borrowing to buy a vehicle.)

There are some discretionary expenses, though, that are not included in either the NRMA or RACV studies but which are worth worth keeping in mind. These include parking, road tolls, car washes and accessories.

WATCH THOSE INVISIBLE COSTS                                                     
                                Toyota Corolla Ascent   Toyota Camry Altise     Toyota Landcruiser GXL                                  
List price (including air-conditioning) $22,990         $29,990         $69,990 
Trade-in value after 5 years            $10,334         $10,347         $31,461 
STANDING COSTS (average $/week) $115.23         $150.92         $294.67 
Depreciation                    $59.58          $87.12          $170.34 
Interest on financing costs             $32.92          $41.99          $96.02  
Rego, insurance, licence, RACV m/ship   $22.73          $21.81          $28.31  
RUNNING COSTS (average per/km)  $17.22          $18.92          $30.58  
Fuel                            $10.46          $13.45          $19.71  
Tyres                           0.97            0.92            $2.12   
Servicing and repairs           $5.78           $4.55           $8.75   
TOTAL COSTS (based on 15,000km/year)                                                    
TOTAL AVERAGE (per/km)          $57.17          $71.24          $132.73 
TOTAL AVERAGE ($/week)          $164.91         $205.50         $382.89 
ANNUAL COST                     $8,575.39       $10,685.93      $19,910.07              
ASSUMES 15,000KM PER YEAR AND FIVE YEARS’ OWNERSHIP 
SOURCE: RACV