I was having a discussion on this topic the other day with MrWhatsit. I was remarking that I’d always heard that running with cruise control on saves on gas. He said that this makes sense for long, flat stretches, but that it seems to him that it could actually wind up using MORE gas on hilly stretches, as your car has to expend more power to stay at the selected speed while going up a hill, whereas normally you might just let your speed drop down by a few miles an hour in that situation.
I said, hmm, that is an interesting point. So I ask you, what’s the straight dope on cruise control? Does it save on gas usage in all situations (I am talking about reasonable situations here; obviously you wouldn’t want to turn on the cruise in stop-and-go traffic or whatever) or only on long flat stretches?
And while I’m at it, would I really be saving gas by going 55 mph in my minivan on the highway, vs. the posted speed limit of 65? There was a recent thread on this, but not with any really definitive answers.
I just did a 200 mile (one way) trip. On the outbound leg I used the cruise control whenever practical and got about 44 MPG. On the inbound leg, I didn’t use the cruise control at all and got about 48 MPG. The trip was mostly flat freeway miles.
I have heard the coast over the hill thing before in threads like this, but try this experiment. Next time you are going up a steep hill, take your foot all the way off the gas and watch just how fast your speed drops. You will find it goes down like a submarine with a screen door.
Now as far as is it helpful to have your speed drop, the answer may be no. I know that sounds counter intuitive, but if your car was working at a very efficient part of the cycle and you drop the speed to where it is no longer working as efficiently you can get worse fuel mileage at a lower speed. I have an MG that due to the cam I put in it gets better fuel mileage at 70 then it does at 55.
But putting all of that aside, my gut feeling is (and I have played with this many times) is that you will get better fuel mileage with cruise, as it never screws up, and drops your speed too far, and then stabs the gas to get back to speed. Sooner or later a driver attention will wander.
jharvey963 it is not unknown for mileage to vary on the outbound and return legs of a trip. I used to be able to drive to college on one tank of gas, but had to stop for gas on the return trip.
It depends on the car and your driving habits. Older cars try harder to maintain the speed. If you go up a hill, they’ll quickly start applying the throttle until the car accelerates to the correct speed. Newer cars are smarter and will just give it a little throttle to maintain a slower speed until there’s less load on the vehicle. They only go heavy on the throttle if it’s a really steep hill or you’re going uphill for a very long period of time.
It is possible to drive more efficiently than either of these systems, though it is possible to drive less efficiently too. I would guess the average person lies somewhere in the middle.
In fuel mileage demonstrations and contests, the driving technique for best mileage is constant throttle - accelerator pedal kept at one spot. Makes for a great TV commercial for Yomofo brand gasoline, but has obvious limitations in the real world with real traffic.
A much more practical technique is constant speed. This is what cruise control attempts to do. A careful driver can do it more precisely than cruise control can, but it’s a pain in the neck to continuously be that attentive.
I’m inclined to think that MrWhatsit is correct, that with significant hills the human will do better than the cruise. It’s analogous to automatic overdrive transmissions, which shift great on most terrain but start acting stupid at a certain degree of hilliness.
As others have noted, data such as this are usually meaningless. The effect of prevailing winds is such that airplanes going between California and Australia can make it one way without refueling, but going the other way must stop to gas up. Motor vehicles are likewise significantly affected by wind, sometimes dramatically so at highway speeds.
Next time, try the outbound leg without cruise and the inbound with, then compare the two outbounds to each other, and the two inbounds to each other. Of course, a deviation from normal wind direction could skew things, but with just one run each way it’s a virtually certainty it’s skewed.
Congratulations on the perfect application of a catchphrase. It certainly deserved the smiley.
As for the second question of the OP, various cars have the optimum speed (or more accurately, RPMs) for different gears. Running slower or faster than this optimum speed will reduce the fuel efficiency. The optimum speed for “Drive” with your minivan may be 47, but 55 would still get better mileage than 65. I had a vehicle once that did better at 60 than at 55 when I checked it, but this may have been the result of other factors mentioned in other postings.
The odds are that you will get better mileage at 55 than at 65, but the other question is: What is your time worth?
For instance, say you take a 300 mile trip. At 55 mph you get 25 mpg and at 65 mph you get 21 mpg. Let’s also assume gas is $3.099/gallon.
For the 55 mph trip you would use 12.00 gallons of gas, it would cost $37.19 and it would take 5 hours 27 minutes.
For the 65 mph trip you would use 9.68 gallons at $29.99 and it would take 4 hours 36 minutes.
Going 55 instead of 65 would save you $7.20 but cost you 51 minutes. The question is if the trade off is worth it. For some people, yes; for others, no.
I would suggest trying it for a while and seeing what your results are. Of course, you run the risk of being the recipient of road rage from the idiots out on the road.
The real key to overall savings is checking your driving habits. A very good way of conserving fuel is to minimize rapid acceleration. Make sure you pull away from stop signs, stop lights, etc. at a reasonable pace instead of stomping on the accelerator. When on the highway, accelerate reasonably and maintain a constant speed as much as possible. And make sure your tires are properly inflated as well (to the high end of what the tire manufacturer recommends; usually stamped on the tire someplace).
I think it largely depends on the driver and driving conditions.
For most drivers the cruise control probably wins, for very attentive drivers the reverse would be true.
On “Top Gear” about a year ago, Jeremy Clarckson drove a new Audi V8 diesel from London to Edinborough and back, on one tank of fuel. He barely made it but one significant detail I noticed was that fairly early into the trip he knocked the cruise control off to save fuel.
Not many of us have his skills and experience so the cruise control would probably have been a better option.
I spot one small problem in the OP. While it is certainly true that the cruise control will “accelerate” to maintain speed on an upgrade, it will also “decelerate” to maintain speed on a downgrade.
On paper this looks like a wash, thought other factors would shade the actual result some.
Thank you for the informed reply! (And all of the other informed replies in this thread, as well.) As far as what my time is worth, it really depends on circumstance, of course. There are times that it’s not a big deal to go 5 mph slower to save some cash, and times when it is.
Yes, but with the cruise control off, the driver himself (or herself) would ease off the accelerator and allow the car to coast down the hill, no? This is assuming an attentive driver that doesn’t just keep jamming on the gas and speeding up on a downgrade, which does happen more often than not, I’d guess.
I suspect that on all but the steepest gradients most drivers keep the same amount of pressure on the throttle all the way up and down. I know all the people I’ve ridden with recently do.
I’ve been driving the 2005 Prius for a couple of weeks now. Round trip to the office is about 206 miles. I use cruise control most of the way, set to 70 mph. Part of the trip is slightly mountainous (Chuckanut Mountains), part is through fairly flat land, and part is mildly hilly. The 20 or 30 miles of city driving is often quite congested, especially between Seattle and Everett. I’m averaging right around 47 mpg.
And your point is? Please explain to us clueless Dopers how the effect of windspeed is as significant on the gas mileage of an automobile as the country of its location.
If you are going to be “cute” or humorous at least be pertinent to the discussion.
This is referring to a very famous set of scenes from Monty Python and the Holy Grail, transcribed here. (Note the relevance of the air speed issue.) As these are quite well known among the large majority of people with even slightly geeky tendencies, RNATB assumed that many people on the Boards would get the joke.
I believe the Monty Python reference to be be de rigueur in these discussions. I’m surprised that those crafty Toyota engineers don’t have a “optimal gas mileage” option within maximum and minimum speed constraints. They could probably use some sort of fuzzy logic technology to determine this.
You are most welcome. I forgot to mention that the information was mostly gleaned from a decade+ of listening to Car Talk on Public Radio. The important thing is to know what are the factors involved and consequences of behavior (as with many areas of life). That way you can make educated choices. I would suspect that many people I see speeding away at stop signs/lights don’t have a clue why they get poor gas mileage.
As you’ve evidently gathered (from your second comment), reasonable human behavior will result in better mileage rates than automatically using the cruise control in hilly conditions. MrWhatsit was correct in what he mentioned as reported in the OP.
I do like to get moving though. Only up to the reigning speed limit but I like to be there in less than 2 seconds.
I’m sure this is largely true. I haven’t really had an opportunity to put the new car to the test. It appears to be fairly uniformly crap (but see above)
The previous one acheived an invariable 32MPG regardless of if I drove it like an old lady or a boy racer. 55MPH or 140, made no damned difference at all. I know this flies in the face of all knowledge and wisdom, even Rick wouldn’t believe it but what can I say?
There something wrong with your figures above. At 65 mph/21 mpg, you will use 14+ gallons to go 300 miles and it will cost you $44 and change. Your point is essentially unchanged, though.
As far as cruise controls go, mine has a button labeled Coast which as far as I can tell, acts like you’ve taken your foot off the accelerator. It’s useful for going down steep hills while in cruise control. I’ve only rarely been able to use it, since I seldom use the cruise control and then mostly in fairly flat terrain.
