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Does cruise control cost more in gas?

  #1  
Old 08-10-2007, 01:08 PM
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Default Does cruise control cost more in gas?

The Toyota Prius cruise control keeps the car within 1mph of the selected speed.
That means when you encounter a hill it hits the gas to maintain speed, and the steeper the hill the more it hits it.

On the other hand, if don't use cruise control, you can keep your foot steady on the gas and allow the car to fade 1 or 2 mph up hill and gain it back on the down hill side. This seems like it would use less gas. on long up hills if you prefer to maintain your intended speed you can slowly depress the accellerator to slowly regain speed, which again seems like it would use less gas than punching it (as the CC does) at the base of the hill.

So what I am saying is that allowing your speed to vary a bit up and down hills would use less gas than mainaining a constant speed.

Am I wrong?
 
  #2  
Old 08-10-2007, 01:54 PM
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Default Re: Does cruise control cost more in gas?

I'm a fan of CC so what I do on hills is just hit the decelerate button 3-7 times as I climb to get a similar effect. Both of my vehicles seem to increase or decrease by 1mph for each tap.
 
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Old 08-10-2007, 01:58 PM
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Default Re: Does cruise control cost more in gas?

It's hard to say. I think for people who are interested in getting good fuel economy, but aren't interested in hypermiling, then CC is good. It will prevent you from gunning the accelerator to get up a hill faster or have some speed creep.

I generally only use CC if my leg/foot is tired from a long drive. At that point I concede any hypermiling rules unless I intervene at appropriate times [such as long downhill sessions]
 
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Old 08-10-2007, 01:59 PM
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Default Re: Does cruise control cost more in gas?

Originally Posted by scm2000 View Post
Am I wrong?
No you're quite right. The "manual" technique you describe for dealing with hills is called Driving With Load (DWL). By keeping RPMs constant rather than speed constant, you can increase your mpg. Of course, if there are cars on the road behind you they'll most likely get angry with all the variations in your speed.
 
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Old 08-10-2007, 04:57 PM
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Default Re: Does cruise control cost more in gas?

Hi,

In the case of the Prius, we have maps of the specific fuel consumption. If the rpm stays below 3,100 rpm, there really isn't a performance hit. But if the rpm exceeds 3,100 rpm, it is best to slack off the speed going up.

My rule of thumb is to look for the heavily loaded truck climbing up the truck lane. He is going up the hill at the perfect speed, fall in behind him at a safe distance and you'll do great.

Try to crest the hill at the slowest possible speed because gravity will soon be your friend. Then when headed on the down grade (wait for it, wait for it) just catch up as best you feel the conditions allow. The battery has a finite capacity and after a couple of hundred feed altitude change, you're not going to get a whole lot more. So enjoy the trip.

Bob Wilson
 
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Old 08-11-2007, 02:23 AM
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Default Re: Does cruise control cost more in gas?

Originally Posted by scm2000 View Post

On the other hand, if don't use cruise control, you can keep your foot steady on the gas and allow the car to fade 1 or 2 mph up hill and gain it back on the down hill side. This seems like it would use less gas. on long up hills if you prefer to maintain your intended speed you can slowly depress the accellerator to slowly regain speed, which again seems like it would use less gas than punching it (as the CC does) at the base of the hill.

So what I am saying is that allowing your speed to vary a bit up and down hills would use less gas than mainaining a constant speed.

Am I wrong?
You're right: This is called driving with load and is an easy way to improve gas mileage over slightly hilly roads. I find that varying by 5-10 mph is a good range. (65 mph - 58 mph - 64 mph - 56 mph, 65 mph, 57 mph...etc)
 
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Old 08-11-2007, 06:17 AM
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Default Re: Does cruise control cost more in gas?

Im lazy - usually just keep the CC on.

for smaller hills, the CC does not rev up much (+500rpm say from 1500-->2000) and my IMA may kick in a little.

for larger hills, I may tap the CC down 1-3 times (1mph each) as the rpms approach 2000. On the downside of the hill, I tap the CC up to get back to target speed.
 
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Old 08-11-2007, 11:45 AM
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Default Re: Does cruise control cost more in gas?

It is likely dependent on terrain.

On the flat at highway speeds, the Prius hangs around mostly in efficient RPM ranges with cruise control. Gentle hills don't seem to change it much. I recently did a test comparing CC to manual pedal control on a 50 mile stretch of highway with gently rolling hills (results soon to be posted somewhere when I take time for some final editing of the writeup). At 60 MPH the results were nearly identical at ~60 MPG.

On steeper hills, CC's aggressiveness in trying to maintain speed tends to push RPM into inefficiently high ranges. I've done better than 60 MPG in such terrain (though not in a test setting) doing what Bob describes on uphills as needed to keep RPM within efficient ranges, and using warp stealth on downhills.

Of course if you don't have a tachometer, you won't know RPM. Here's a rule of thumb I have proposed, validated by a small handful of others, for those without added instrumentation: Keep the instantaneous MPG above half the vehicle speed during acceleration if possible. I've found this keeps RPM below ~2400 (measured by ScanGauge and Can-View) at highway speeds.
 
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Old 08-11-2007, 12:40 PM
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Default Re: Does cruise control cost more in gas?

Originally Posted by JimboK View Post
Of course if you don't have a tachometer, you won't know RPM. Here's a rule of thumb I have proposed, validated by a small handful of others, for those without added instrumentation: Keep the instantaneous MPG above half the vehicle speed during acceleration if possible. I've found this keeps RPM below ~2400 (measured by ScanGauge and Can-View) at highway speeds.
Anyone know if this holds for the TCH as well? I find on ~6% grades I have to give enough throttle that the instant MPG hovers around 20 mpg to hold my speed with the TCH.
 
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Old 08-11-2007, 06:35 PM
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Default Re: Does cruise control cost more in gas?

Originally Posted by nash View Post
Anyone know if this holds for the TCH as well? I find on ~6% grades I have to give enough throttle that the instant MPG hovers around 20 mpg to hold my speed with the TCH.
Hmm, good question. I really don't know. The rule is specific to the Prius. I haven't taken any time to get acquainted with TCH's performance characteristics.

But in the Prius as well, I suspect that holding speed on a 6% grade would cause iMPG to drop considerably lower than MPH/2. On my trip to Hybridfest last month I drove on some mountain roads that I'm guessing were in that range (and some steeper), and I either had to set the pace of a semi or let iMPG go in the toilet. I chose the former when traffic allowed; it allowed more time to enjoy the scenery.
 

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