Does cruise control save fuel?

By Stuart Johnston 

The answer to this question is a complex one.  The best way to save fuel is to drive at the slowest speed possible in the highest gear possible, at a constant throttle (accelerator) opening. The more you depress the accelerator pedal, the more fuel will be pumped into the engine.

But it is not always practical to drive, for example, at 80 km/h in sixth or seventh gear  on a freeway, where most of the other traffic is doing its best to maintain or exceed our national speed limit on freeways of 120 km/h.

Good for clear, open-road driving

Cruise control is a tool best used for freeway driving , or on dual lane open road driving, when there is not too much in the way of traffic. In heavy traffic, the speeds of cars around you will often cause you to suddenly slow down and speed up, and here it is better to manually control the accelerator pedal.

The most important aspect of cruise control is that it prevents you from inadvertently increasing your speed on long open stretches of road. Today, powerful modern cars are often configured to cruise perfectly safely and effortlessly at speeds of in excess of 160 km/h. This is particularly true of the more expensive German cars, which are designed for operation on their autobahns, where speeds of up to 200 km/h are not uncommon.

Driving such powerful and efficient cars, which feel perfectly safe at speeds of 200 km/h, in South Africa, it is all too easy on long stretches of open road to have your speed creep up to considerably more than the speed limit of 120 km/h. Setting a cruise control speed of 120 km/h will prevent you from inadvertently breaking the speed limit and risking a heavy fine.

But Cruise Control doesn’t anticipate up-hills and downhills.

In terms of saving fuel, Cruise Control works best where the terrain is reasonably flat. However, on roads with major accents and descents, Cruise Control cannot anticipate the road ahead the way an alert driver can.

An alert driver will know that the best way to save fuel is to be as light on the accelerator as possible, while keeping the car in a gear as high as possible for the terrain to reduce engine revs.

In the case of undulating terrain, the alert driver will allow the car to speed up slightly on down-hills, and carry that momentum through to uphill sections, without having to alter the accelerator pedal positioning, or manually change down a gear or two.

In Cruise Control mode, the car’s computer will simply “instruct” the car to maintain a selected speed, and on downhills this can actually mean the car is braking, while on the following uphill it will then not take advantage of momentum that a driver can have dialled in.

Deciding when to accelerate harder, or downshift for up-hills

Of course in extreme up-hills, the driver will need to downshift a gear or more to maintain momentum. Here it is a judgement call on whether to simply push down the accelerator harder to maintain speed, or keep a constant accelerator position and manually downshift to a lower gear.

Tests with fuel flow meters have found that, generally, it is better to gradually depress the accelerator to maintain speed on up-hills, but not to labour the engine in a higher gear. The trick is, with speed dropping off uphill,   to down-shift one gear at the optimum point, so that the engine remains in its optimum power range, at which point you can actually ease off the accelerator pedal to maintain speed in the lower gear.

In Cruise Control, the car’s electronic control units will be prioritised to maintaining that set speed regardless of fuel usage, or gear selection, and this will mean that it will actually use more fuel than necessary, in an undulating terrain.