Jim and I have an ongoing ahem debate about using cruise control in the car - he claims it uses an incredible amount of gas, and I figure if it actually does use any extra, it’s not enough to offset my feet staying comfortable while driving long distances.
FTR, my car is a 91 Sundance with electronic fuel injection, but not much else electronic on it (as far as I know).
Anybody know the skinny on cruise control? Does it really guzzle so much gas that it makes a huge difference on long road trips?
One of the advantages of cruise control, apart from the absence of speeding fines, is the reduction in fuel consumption achieved by the regulation of you speed.
I know this from experience with a V6 Mitsubishi Magna and lots of highway driving. Check this and Google for others.
Well, Jim doesn’t seem thrilled to have his cherished idea called into question, but thanks for the input. I have used cruise control and found my gas mileage to be just fine; Jim says that with a car his family had, if you used the cruise control you could almost watch the gas gauge going down. I suspect it was a problem with this particular car, not cruise control in general. Or perhaps older models without fuel injection didn’t operate as smoothly with cruise control on.
All the cruise unit does is monitor the car’s speed, and yank on the car’s gas pedal with a cable to keep the car at the desired speed. Really, that’s all it does. The car still has the same engine, and therefore, still uses the same amount of gas.
Of course, there are a few other necessary connections as well: The unit also watches the RPM (speed) of the engine, so it won’t get over-revved by accident. And the cruise gets cancelled if the brake and/or clutch is pressed.
It is considered pretty obvious that a car is most fuel efficient when running at one steady speed, rather than constantly accellerating and slowing back down.
If a driver has trouble keeping the car at a steady speed, cruise control use will increase fuel mileage.
If a driver can somehow keep the car at the exact same speed throught the trip, cruise control will make zero difference, as that’s all the cruise unit does anyway.
By the way, no cruise control is 100% perfect; it’s a compromise between smoothness and accuracy. You don’t want the engine to be revving up and/or coasting every time the car loses 1mph of speed, because that would be annoying. On the other hand, if the cruise allowed, say, a 10mph range of operation, that would defeat its intended purpose. A good engineer will make a design that is smooth, but with minimal speed fluctuation.
You can beet the gas milage of CC by acceleratng slightly before you get to a hill then let up while climbing it and accept the speed loss, and by letting up before you get to a downgrade. But these are small gains (unless you are just driving hills ).
CC gets inefficent if it is trying to maintain a speed up an incline that is just doesn’t have the power to maintain (or just barely) - rare but it does happen.
Best efficiency is gotten at constant speed. Opening the throttle results in operation of the accelerator pump, or its equivalent on fuel injection engines, which puts a shot of fuel into the intake. The cruise control maintains a more constant throttle setting than most people can manually (or is it pedually?) and probably results in slightly better fuel economy.
Manual throttle control is no better than cruise control when going up a hill that is too steep for you engine to maintain speed on. I don’t know about you, but when I go up such a hill I keep pushing down on the throttle until it is wide open, just like the cruise control would and probably not as smoothly.
k2dave is saying that instead of maintaining constant speed on a hill, you slow down while going up hill. If you don’t slow down your car has to go into a less fuel-efficient range of engine operation. If you leave a cruise control on that keeps the speed constant going up hill, it will operate in this bad efficiency range, where as a manual driver can compromise the speed and achieve better fuel-efficiency.
Similarly, if you speed up on the way down the hill you can move at a faster speed while maintaining fuel-efficiency. The cruise control will not do this either.
I believe the most fuel-efficient way to cover a route is to keep the car in a sweet spot on the power-to-fuel consumption curve. That is definitely different from constant speed in hilly terrain.
Perhaps new computerized cruise-control automatic transmissions are optimized for fuel efficiency. I don’t know.
ok David Simmons a very quick example on how it is entirely false.
Lets say you drive at a constant speed 60mph but on the way there is a hill where you will (or CC will have to ) floor the accelerator to keep 60mph. You will use a lot more gas then allowing a loss of speed to lets say 40mph and not having to press down on the gas nearly as far.
Or even a better example - lets say you are traveling at 10mph but there is a hill where you will have to brake to maintain 10mph instead of coasting up to 25 mph.
Speed of the car is only one of multiple factors (actually it’s a result of other factors) that can be used to determine efficiency. Some of the more important ones have to do with engine/fuel system design, incline of the car, wind, car weight, load on the engine (electrical, a/c, etc.), tire pressure, etc. etc…
Your statement can only be true when there is no wind, no incline, no varying demand on the engine, constant weight of the car (remember you are burning fuel and the car is getting lighter), no turns (or at least a constant turn), for that matter no rotation of the earth unless you are traveling exactly east or west.
***I did have something here that was rather harsh. This is because I misread your comment above. As I was waiting for this to post I had a chance to re-read your comment and see how this was inappropiate. If it did post I’m sorry and withdraw it. ***
Let me ask you something. If an engine is at constant rpm’s, is fuel usage constant?
I think maybe my statement should have been something like “best efficiency comes from constant engine RPM.” If that RPM happens to be at the peak of the efficiency curve that is. I know that diesel trucks run their engines at constant speed. They run about 18-1900 RPM. If the engine speed starts to drop because they are going up hill they shift down and vice versa. Railroad diesel electrics are also run at constant speed when enroute.
Fuel mileage will, of course, varies with a lot of environmental conditions, but if you stay at the peak of the efficiency curve you are doing the best you can. It seems to me that cruise control can do that better than 99-44/100ths % of drivers.
Again best efficency is not from constant rpm’s but it will vary depending on the load of the engine - but it is closer and a good guide rule. and yes normally CC will beat most drivers in mpg but any driver can beat CC if they try.
I remember reading about mileage competitions (I think the gasoline companies used to do this). Maximum gas mileage is achieved with constant THROTTLE (put the pedal in one place and leave it there). As you can imagine, this can be problematic with hills and is essentially impossible when driving in traffic.
Practically speaking, the best technique is constant speed, with some common-sense modifications–avoid using the brakes and do not accelerate vigorously.
As to the original question, sorry Jim, but I’ve never heard anything to corroborate what you describe. I’m even having trouble trying to imagine a problem or scenario that could do that.
Well, maybe it was a small engine in a big car, coupled with an automatic gearbox, and driven in a hilly area. Lots of shifting (which some underpowered CC’s will do) can lead to a loss of fuel efficiency.
No, never. Cruise controls are designed for driver comfort, relaxation, and safety. There’re few things more dangerous on a highway than a driver that constantly changes his speed!
Cruise control is typically an option on lower-end (read “cheap”) cars. When it’s standard, it’s a higher-end car. Hence, if you can afford cruise control, reason the manufactures, you ought not worry about $0.05 worth of gasoline to accellerate up a hill.
Trucks and RVs being the general exception due to gravity, most of my hill experience (admittedly anecdotal) indicates the humans drive like a cruise control – they give it more gas so that they don’t slow down up a hill. Then they break or downshift on the way back down. This of course assumes we’re not talking curvy terrain where a decrease in speed is justified.