Road and car questions you’re too embarassed to ask

Moving Well | RACV | Posted on 13 August 2019

Your most commonly asked driving questions, answered.

Whether you're new to driving or consider yourself a roads connoisseur, sometimes a road rule or motoring query will leave you stumped. So to help you keep your car in mint working condition, give you an insight into fuel price cycles and debunk social-media spawned driving myths (cruise control aquaplaning, we're looking at you), these are your top driving questions, answered.

Car fuel gauge with petrol light on to show tank is empty

Why doesn't diesel ever get as cheap as petrol at the low end of the cycle.  

When you take transport companies with supply deals out of the equation, far less diesel is sold to regular motorists than petrol. The diesel market is, therefore, less competitive and not subject to a price cycle like petrol. So petrol prices at the lower end of the price cycle will often be lower than diesel. 

Nevertheless, a diesel car usually offers considerably better fuel economy due to it being 20 to 30 per cent more efficient than an equivalent petrol engine. On average the fuel costs in your diesel will still be less than a similar petrol model. 

However, there are additional costs with diesel that you should be aware of. Firstly, many manufacturers charge a premium for diesel models which they claim is for the more robust design required. Servicing costs can also be slightly higher and you may need to use a fuel additive to reduce emissions. 

How do I top up my water? 

That depends on what you mean by water. Windscreen washing fluid does run out and topping it up is straightforward. A reservoir for this under the bonnet will be clearly marked. Top this up with water or use an additive for more effective cleaning.  

Coolant, on the other hand – the fluid used to remove excess engine heat via the radiator – is a no-go area for motorists without expert knowledge. Tamper with the cooling system and you could burn yourself and risk seriously damaging the engine. Manufacturers are particular about the fluid they use and it will never be just water. Coolants have anti-freeze, anti-corrosion and lubricating properties, and the concentration varies by manufacturer and operating conditions. The coolant will need changing occasionally and the hoses need to be checked, so service your car regularly. 

How often do I need to replace my tyres?

Knowing when to replace your tyres is critical to safety. The speed at which they wear depends on how hard the car is driven during cornering, braking and acceleration, how heavy the car is and how soft the tyres are. Generally, the smoother the driving, the longer the tyres last.

Repeated heating and cooling of a tyre due to these forces affects the structure of the rubber, particularly how it springs back into shape. The tyre hardens, which degrades its grip and makes older tyres more prone to punctures. All tyres have in-built tyre wear indicators to show when they are worn too far. Look for small, raised squares set in the bigger grooves of the tread. When the face of the tyre wears down to this point, it's time for a change.

Maintaining correct pressure will prolong a tyre's life. All car-makers recommend tyre pressures for their cars, listed on a small placard usually found inside the driver's door jamb or fuel cap. 

I've read on social media that using cruise control in the wet causes aquaplaning. Is this true? 

This urban myth has been circulating since at least 2001 (thanks a lot, Facebook). We've looked into this claim - that using cruise control in the wet can cause a car to lose grip due to a thin film of water or ice and increase its speed - and can't find support for it. 

If a car hits water and loses grip, the wheels would keep rotating but the vehicle would slow. If the wheels did spin, the cruise control would think you've accelerated and reduce throttle since it reads the speed of the wheels, not the body. In any case, a touch of the brakes will disengage the cruise control. Stability and traction control systems would intervene in moderns cars. 

That said, drivers should be cautious when using cruise control in inclement weather. Cruise control doesn't take into account worsening conditions, which risks driving at dangerous speeds. 

Why is it bad for your engine to drive on an almost-empty fuel tank?

Conventional wisdom is that it's better to keep a tank at least quarter-full at all times, the rationale being that running a tank too low means picking up the dregs that contain dirt and other particles at the bottom. The advice is good, but not for that reason. Fuel is always taken from the bottom of the tank, which is often in a secondary depression feature to ensure there is sufficient depth of fuel around the pick-up when it is near empty. So any detritus at the bottom is being picked up whether the tank is full or empty. This is why cars have fuel filters. 

The real reason running on empty is bad for you is that you risk running out of fuel. This can be bad for cars, particularly for fuel-system injectors and fuel pumps. Even worse, cars running out of fuel perform erratically, which can be extremely dangerous in traffic.