Cruise control is a useful tool for drivers of both motorhomes and tow vehicles as well as the family car. The primary purpose of cruise control is to maintain a preselected speed and relieve the driver from having to constantly maintain pressure on the gas pedal. On long drives that pressure often results in leg cramping and stiffness which can not only show up in the leg on the accelerator pedal, but is also often transmitted up into the buttocks and lower back. Using cruise control can allow the driver to be a little more relaxed. To the extent that relieves stress, its a good thing, but you certainly don't want to become so relaxed that you get apathetic or drowsy! Avoiding leg cramps is definitely a good thing. Using cruise control is said to improve gas mileage too. Generally it does since most people can't or don't maintain steady pressure on the accelerator and the cruise control does.
Cruise control can usually help get better fuel economy. Each time you press down on the acclerator pedal the fuel system dumps gas into the engine. You might be surprised how often you press down on the accelerator, even when you are trying to simply maintain a steady speed. Cruise control minimizes these surges and allows the engine to operate more efficiently. However, cruise control can be problematic when towing a heavy trailer or negotiating hills. Many drivers of heavy vehicles like to "get a run" at steep or long hills and cruise control cannot anticipate upcoming hills. It can only react when the hill has already slowed you down. Drivers who routinely maintain a steady foot on the accelerator won't see as much mileage improvement as those who tend to surge and back off frequently. I once had a construction supervisor whose inconsistency on the gas pedal was enough to make his passengers car sick. It was enough to cause his passengers to rock forward and back rather strongly. Fortunately I wasn't susceptible to motion sickness but other employees were. Riding with him was not comfortable. He would have seen significantly better mileage by using cruise control (and probably avoided passengers vomiting in his truck!).
Why does cruise control improve gas mileage? For one thing, each time you press down on the accelerator it dumps extra gas into the engine. Avoiding frequent and often unnecessary movement avoids this extra fuel usage. Some years ago I recall seeing something called an "Econometer" that supposedly helped drivers improve gas mileage. It was a dial with green, yellow, and red segments and the goal was to keep the needle in the green as much as possible to get the best possible gas mileage. How did this work? Well, what it really was, was a vacuum gauge. Low vacuum occurs when the engine is under load, so maintaining a steady throttle (high vacuum) and avoiding putting load on the engine improves gas mileage. Most drivers will get better mileage using cruise control but there are a few professional drivers who have actually been able to get better mileage without it. So, you may hear stories about cruise control not delivering as advertised but you will want to test your own vehicle and your own skills before you make up your mind for your situation.
Cruise control should NOT be used on wet, snowy, or icy roads. It should be apparent that you shouldn't use it on snowy or icy roads but some people have tried to use it in the rain, usually much to their dismay! Ithas been demonstrated that cruise control, attempting to maintain speed on slick or even just roads, can sometimes cause you to loose control of your vehicle. Even a little rain can be enough to cause problems. The coefficient of friction between your tires and wet road is about half of that on dry roads. In addition, vehicles tend to hydroplane on wet roads. When that happens you aren't driving on the pavement, you are driving on top of the water on the pavement and you have very little traction and even less control. Because of lack of traction, the speed doesn't increase as it normally does when the cruise control opens the throttle, so it opens it more and more and when it eventually does get traction, the reaction is sudden and much more than needed, sending the vehicle out of control. The classic debunking website snopes.com lists this as TRUE. So, if it starts to rain or snow, or even if the roads are just wet, shut off cruise control!
It is not usually a good idea to use cruise control when driving in hilly country. The cruise control cannot anticipate upcoming grades the way a human driver can and will NOT downshift to help maintain a safe speed when descending hills. The cruise control is likely to be constantly having to adjust when operated going up and down hills, completely negating any mpg savings, causing unusual wear and tear on the components, and possibly causing unwanted changes in speed. I once had a cruise control cause an unwanted downshift that jerked a trailer so hard it snapped the hitch pin! Fortunately the safety chains kept the trailer from getting away and we were able to stop safely and get a new hitch pin. I now avoid using cruise control when pulling a trailer! Many vehicle manufacturers discourage using cruise control when towing, except on flat stretches of road.
I've seen drivers use the cruise control buttons like manual driving controls instead of using the gas pedal. While I have no hard evidence that this isn't a good thing to do, common sense tells me it isn't. There is usually some delay as the cruise control responds to input to accelerate or slow down, a delay the "real" driver doesn't normally introduce and I seriously doubt such use will provide the fuel economy benefits of setting the cruise control and leaving it alone. If nothing else it puts extra wear and tear on all the components of the cruise control and could cause premature failure. I do not recommend using cruise control in heavy traffic. The speed changes too often and the potential for another vehicle suddenly cutting you off and forcing you to slow quickly is too great. Cruise control cannot anticipate nor even react to such things. Also using cruise control in this manner puts unusual loads on the switches and motors that make it work, possibly causing malfunctions and/or premature failure. Cruise control is best used on the open road when traffic is light and/or flowing smoothly.
Cruise control is usually installed as a factory option. There are companies who make aftermarket kits that are reported to be fairly easy to install and not too expensive. Just replacing an existing cruise control switch will run around $150 in labor so that gives you some idea of the starting place for the cost or labor involved in installing a new kit. If you are looking for the cruise control to improve gas mileage to save money, be sure to count all the cost of parts and labor to figure out how long it will take for it to pay for itself. However, if you want it for convenience or to relief some of the muscle cramps of long road trips you don't have to worry about whether the mpg savings will be enough. Just count any savings as a nice bonus!
There are many anecdotes about naive drivers treating cruise control like an "auto pilot". Hopefully, none of them are true, but in the interest of a little humor I will retell the story of the RV driver who wrecked his brand new motorhome after he set the "auto pilot" and went back to make a sandwich or a cup of coffee! Other variations include the driver who got in the back seat of his sedan to take a nap and the woman who went back in her van to care for her crying baby after setting the cruise control. Not surprisingly, snopes.com lists these as LEGEND. They are, however, useful to warn people about the using their cruise controls correctly.
There are devices sometimes dubbed "cruise control" for ATVs. In reality they are just devices that lock the throttle in position. As long as you ride on level ground you will maintain a relatively steady speed but it won't adjust like a real cruise control when you go up or down hills. You will slow down going up hills and speed up going down. ATVs with such a device should also be equipped with a "deadman" kill switch to shut the engine off if the rider falls off so the vehicle doesn't continue speeding away. Some early automobiles, like the Model A Ford had throttle lever on the steering column that could be set to maintain a constant throttle but you won't see them on modern vehicles.