Car dealers have a habit of trying to sell you stuff you probably don't need after you've already laid out a small fortune or taken out financing for a brand new car.

Some optional extras – premium paint colours, alloy wheels and spoilers, for instance – are a matter of personal taste ... or sign of a midlife crisis. If that's how you want to roll and you're willing to pay extra, so be it.

But the dealer may push other options that sound like they should have been included in the first place, such as rustproofing, paint protection, extra airbags or an extended warranty.

Are any of these really necessary or worth the money? Sometimes, but not very often.

Wheeling and dealing

It seems that every car dealer has gone to the same salesmanship school – they tend to throw in a few optional extras to sweeten the deal and whet your appetite for more add-ons.

It's important to know the dollar value of what they're spruiking. Before you go shopping for a new car, check the manufacturers' websites. Most list the available options with prices and let you experiment with configuring a car in different ways. It may turn out that the sales sweeteners aren't so sweet.

With a little advance online work, you'll be in a much better position to negotiate a deal for any extras that you actually need – or at least want – when you step in to the showroom.

Unnecessary new car extras

Extended dealer warranties

Extended dealer warranties can sound great in the showroom, but the devil is in the detail. Dealer extended warranties often require you to have the car serviced exclusively by the dealer who sells you the car, and missing a scheduled service can void the warranty. They can also be very restrictive in what they cover.

Extended factory warranties are usually less restrictive than dealer warranties, but you still need to read the terms and conditions carefully before signing. In most cases the standard three- or five-year warranty is good enough.

Rustproofing and paint protection

Rustproofing and paint protection raise the question: isn't the car already rustproofed? The answer is yes. New cars are treated at the factory and any rust problems should be covered by the warranty. Unless you live next to the sea or like to drive along the beach, don't opt for extra rustproofing.

Paintwork is also covered by warranty for major problems and shouldn't need anything more than an occasional clean and polish. Dealers often charge several hundred dollars for paint and rust protection; if you really want this extra treatment, you can get it aftermarket (or even do it yourself) for much less.

Fabric protection

Fabric protection might consist of nothing more than the dealer treating the car seats with a can of fabric protector from the supermarket. You can do that yourself for a lot less and with just as good a result.

Alarms

Alarms are of questionable value. It's likely to be ignored if it goes off, and you could even be fined for noise pollution if it goes off without good cause. All new cars have engine immobilisers, usually with central locking, and that's all you need. If you want a fully featured alarm system, compare the dealer price with aftermarket options; you might get a better deal elsewhere.

If you want to know how steal-able your model of car is, see our article on NRMA's car security ratings.

New car extras to consider

Some accessories can be useful and appealing additions to your car, but you'll probably get them cheaper – sometimes a lot cheaper – at auto stores rather than through the dealer.

However, some extras must be factory-fitted, or you might just want the convenience of having the dealer supply and fit the option for you. Also, dealer and factory-fitted accessories are usually covered by the vehicle warranty. It's your call.

Floor mats

Floor mats help keep the carpet clean, but do you really want to pay anything from $70 to more than $100 for them when you can get aftermarket car mats for $40 or less? The same applies to seat covers and boot liners.

GPS navigation

Factory-fitted dash-mounted systems offer bigger screens and integrated functionality and are less easily stolen than portable models (although car thieves sometimes target these high-end systems and cause extensive damage to the dashboard in the process). You might also be forced to get your software and map updates from the car manufacturer. Some manufacturers and dealers also offer portable GPS systems, but they're usually cheaper elsewhere.

Other extras you just might need

Headlight and bonnet protectors are really only useful if you often drive on unsealed roads.

Nudge and bull bars aren't necessary for typical city driving and increase the risk of injury to others in a crash. But if you opt for one, a factory-fitted bar has the advantage of having been tested with the vehicle's airbag system – the bar affects the vehicle's behaviour in a collision. If you go for an aftermarket nudge bar, ask the supplier if it's compatible with the airbags in your car.

Premium sound systems can make travelling much more pleasant, but have a listen to the car's standard system first. It might be all you need, especially if it already has a CD player and MP3 player connection.

Roof racks and roof luggage boxes are almost always cheaper aftermarket.

Special suspension such as sports (firmer springs and lower ride) or country suspension (with increased ride height) is useful for some drivers, but test-drive both the special version and the standard suspension so you know what you're getting before you commit yourself to this option.

If you're after a towbar, get quotes from a couple of independent towbar installers to compare with the dealer price. The dealer price will probably be higher.

Window tinting is already standard on most cars. Do you really need even darker windows?

New car extras you probably need

Safety options are always worth having, and usually have to be factory fitted, so you need to choose which ones you want before buying. See our report on ANCAP crash tests to see the difference safety features can make.

Airbags: If side and curtain airbags aren't standard in your chosen model, they may be available as options for a few hundred dollars extra.

ABS: An anti-lock braking system is usually standard on new cars, but can be optional on cheaper models.

Stability control (also known as Electronic Stability Program and Dynamic Stability Control) is an anti-skid technology that applies braking to individual wheels to stabilise the vehicle when it detects a situation such as fishtailing.

Full-size spare tyre: Space saver and temporary-use spare wheels are common, even in big cars. They're only for emergency use, to get you to the nearest garage. If you regularly drive long distances, especially in country areas where there are not as many garages nearby, a full-size spare tyre is a better option.