Ask an expert

What are the disadvantages of putting an auto car into neutral while cruising down a hill etc for fuel saving purposes?

What are the legal aspects?

I can still brake and steer the car in neutral so consider I have it under full control.

Stuart

From the "Ask Jack" archives - 12 October 2008

It is not a recommend practice as I don't believe the gains are as great as what you would imagine (if any), let alone the safety aspects of the exercise.

It depends on the vehicle but let's assume it has a fuel injected engine (which all modern vehicles are). The engine management system is constantly monitoring vehicle and engine speed, throttle position, engine temperature, gear selection (in the case of an automatic) airflow into the engine etc. This is all designed to provide a balance of power and economy over different driving conditions and temperatures.

Throttle position has been highlighted as the one of the biggest contributors to either increasing or decreasing fuel consumption and is the one item the driver has complete control over. Not only can the driver control the speed of the vehicle but they can also control the amount of fuel used by operating the throttle smoothly.

When you travel down a hill in a vehicle with a fuel injected engine and lift your foot off the accelerator pedal, you induce fuel cut to the engine via the engine management system. In other words there is no or absolute minimal fuel entering the engine. This is designed to save fuel and reduce emissions. The moment the throttle is used the fuel is re-introduced into the engine at levels required for the vehicles speed and throttle position etc.

If you simply placed the gear selector into neutral the engine needs to remain idling and therefore requires fuel to be injected. There is absolutely no benefit to be gained in this condition.

The engine and transmission management systems in the more modern vehicles are also designed to help brake or slow the vehicle when driving downhill by automatically selecting an appropriate gear to suit the occasion. Once again vehicle speed, throttle position and brake pedal application are being monitored.

It is considered an unsafe practice to have no engine braking available in any vehicle auto or manual when travelling down hills.

While all modern automatics have fail safes build into the electronics to stop the wrong gear being selected, I have seen the consequences and damage caused by earlier transmissions being placed into the wrong gear when the vehicle is in motion. Obviously skipping neutral and moving from Drive to reverse leaves a nasty mess inside the transmission. Taking your eyes off the road to ensure the correct gear is being selected is also not recommended.

I'm sure most automatics specialists would tell you also that constantly moving a transmission from neutral to drive is not a good practice. Transmissions rely on oil pressure to clutch packs or servo units to operate and would much prefer to have a steady and consistent pressure applied.

Legally I don't believe there is a specific rule that says you cannot drive in this way, however if you were to sit a driving test and carried out this practice I'm sure you would be reprimanded by the testing officer and it is highly likely you would fail the test in my opinion.

Stay in gear and work on throttle control is my best advice.

Definitely. Either in a manual or auto, if you are going downhill and using engine braking (changed down a gear), even if the engine is revving higher (eg 2500rpm) it will not be using any more fuel than at idle anyway and like you said, "When you travel down a hill in a vehicle with a fuel injected engine and lift your foot off the accelerator pedal, you induce fuel cut to the engine via the engine management system. In other words there is no or absolute minimal fuel entering the engine."
So it is either the same or better leaving it in gear than putting it in neutral if your foot isn't on the accelerator. Couple this to increased safety and less wear on the brake pads, it is obvious to leave it in gear.
Do not try changing down and red lining the engine though!

I have a 2004 Subaru Outback 3.0 Ltd automatic (5-ratio) and I most definitely do not notice any attempt by the engine management system to help with engine braking. When going down a steep hill near my house, if I left the car to coast I'd be in top gear and doing a ridiculous speed by the time I get to the bottom.

I manually select 2nd gear to hold a nice 40-50kph downhill speed to avoid riding the brake. There is a tiptronic gate on this gearshift so there is no risk of slipping it into reverse.

Of course you need to select a lower gear to get engine braking. If you went down it in 5th gear in a manual it would still run away. The auto tries to select the most appropriate (fuel efficient) gear for the revs and load on the engine and speed. If you want engine braking and select a lower gear, it will not increase consumption though due to what has already been said.

I have tried this in a modern vehicle with dashboard fuel consumption display... Coasting downhill In gear, with no thottle applied, at about 3000rpm it shows 0.0 liters per 100km. As soon as I put my foot on clutch and revs dropped the computer started in inject fuel, to maintain idle revs, and the display showed (from memory) 1.0 liters per 100km. Not massive fuel consumption,but fuel that does not need to be burnt none the less.

So based on my non scientific test - leave it in gear.