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Old 08-04-2008, 09:09 AM   #1
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Cruise control MPG controversy

My brother says he gets better gas mileage with his Prius when he uses cruise control. I attributed the better gas mileage was on account of the
vehicle being a hybrid. But then, my lovely, sweet tempered, charming,
ex tried to explain to me that she gets better gas mileage using cruise control on her gas guzzler. Her point was that the cruise control prevented her from driving at 80 miles an hour up hills. That makes perfect sense to me. But I was always under the impression that MPG suffered when cruise control was activated. What is the deal? Dave
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Old 08-04-2008, 09:33 AM   #2
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Although I am not a big fan, and don't use it as often as I could, I have found the cruise control to be more efficient when it comes to mileage. Ron
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Old 08-04-2008, 09:55 AM   #3
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I'm a fan of cruise control but my reasoning is a bit different from the 80 mph up a hill reasons. I remember back when cars had carboraters and to reduce response time between when you push the gas pedal down and the engine starts to speed up the manufacturers added accellerator pumps which would squirt a small amount of gas so that response time was instant. Push the accellerator down over and over and you would send multiple squirts of gas to the combustion chamber and gas mileage suffered. It is my belief that the same principle has been carried over to the electronic fuel injection systems. When you first push the gas pedal down, a little extra gas is shot into the combustion chamber to give you quick response. As you cruise along you can ease up on the gas just a bit to maintain the same speed. Every time you hit a good bump in the road your foot presses the gas pedal down just a bit causing a little extra gas to be sent to the combustion chamber for just an instant resulting in lowered miles per gallon. It seems logical that the cruise control would eliminate these little bumps and jiggles in the gas pedal, resulting in a steady position and lean, smooth, even flow of fuel and thus, higher miles per gallon.

The proof would come from knowing how the electronic control system is programmed.

To me it makes sense, but then I haven't had all my coffee this morning.
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Old 08-04-2008, 10:17 AM   #4
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My '06 diesel truck will get better milage with the cruise off, if I allow it to slightly vary the speed on hills. On the other hand my '91 Jag's cruise seems to jump on and off the throttle for small adjustments but it still beats my best MPG when I try manually controlling the speed.
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Old 08-04-2008, 10:20 AM   #5
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My 08 Sequoia gets better millage on cruise, but I don't like to use it while towing.. Just makes me a little to relaxed, and you never know what is gonna happen.....
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Old 08-04-2008, 10:26 AM   #6
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In our 4-cyl (2.4L), 5-speed manual Accord, the cruise puts in a good additional amount of fuel pedal when climbing hills. In level cruise, the pedal is barely depressed at 65mph.

In our 8-cyl (5.7L), 3-speed automatic Travelall, the cruise doesn't put near the additional amount of fuel pedal in when climbing. Of course, it's putting in a good amount already in level cruise.

The Accord has a tendency to overshoot the desired speed when maintaining speed through hills. The Travelall is consistent on speed through hills and does not "overspeed" when cresting a hill.

I think the difference in engine sizes is the key factor here. Greater capacity to perform additional work with a similiar amount of energy.
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Old 08-04-2008, 10:56 AM   #7
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Quote:
Originally Posted by Bob Thompson View Post
...

The proof would come from knowing how the electronic control system is programmed.

...
I agree totally.
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Old 08-05-2008, 12:33 PM   #8
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Our Ridgeline does better with the Cruise on, towing or not.
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Old 08-05-2008, 03:23 PM   #9
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On a technical level I can't see many reasons why the fuel economy would change whether under cruise control or not. Under closed loop conditions the air/fuel mixture is 14.7 to 1 on nearly all gas engines. The ignition timing at various engine speeds vs. MAP would have no reason to change under cruise control Perhaps auto transmission shift points could be more conservative under cruise control-keeping high gear engaged longer prior to a part throttle downshift-since throttle "response" isn't as much of an issue.

The Prius uses an unconventional drivetrain and I haven't thought out all the possibilities there.

At least on my vehicles I can't see a difference. On my car (Camry) I use a scan gauge and specifically checked this out.
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Old 08-05-2008, 03:55 PM   #10
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Quote:
Originally Posted by tpi View Post
On a technical level I can't see many reasons why the fuel economy would change whether under cruise control or not. Under closed loop conditions the air/fuel mixture is 14.7 to 1 on nearly all gas engines. The ignition timing at various engine speeds vs. MAP would have no reason to change under cruise control Perhaps auto transmission shift points could be more conservative under cruise control-keeping high gear engaged longer prior to a part throttle downshift-since throttle "response" isn't as much of an issue.

The Prius uses an unconventional drivetrain and I haven't thought out all the possibilities there.

At least on my vehicles I can't see a difference. On my car (Camry) I use a scan gauge and specifically checked this out.
This is my take on it...

Only possible way I could see a FI system getting worse mileage on cruise would be in the hills where it is accelerating to maintain set speed. If a person is driving they may let the speed roll off a bit and then speed up on the downhills.

Aaron
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Old 08-05-2008, 04:23 PM   #11
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That sounds logical. I've had my cruise control force a downshift on an uphill at 70 MPH where if I was operating throttle, I'd accept a speed loss to avoid the downshift.

In my vehicles the cruise control uses stronger throttle motions than I'm accustomed to using while steady cruise.

All in all though its the braking (or excessively high cruising speed) which kills fuel economy.
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Old 08-05-2008, 04:44 PM   #12
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Having just completed our 4000 mile trip to Oregon and back I found that I did get better gas mileage using cruise control but with a few caveats:

1. The better mileage comes to you as an average over longer distances and greater periods of time (i.e., the longer you keep the rig moving at a consistent RPM the better the mileage). On days that we drove 150 miles or more between stops our mileage was better.

2. Best to use it on 4-lane+ interstate settings so that you can pass slower traffic when necessary without braking or accelerating (the consistent RPM thing again).

3. Don't just set it and forget it. When your rig begins to ascend a hill which requires a down-shift to maintain speed you blow some of the fuel savings you've earned prior to the hill. On milder hills you can reduce your speed just a bit (use the "coast" button on your cruise control) and often keep your momentum up the hill without forcing a downshift. I found that my F-150 and 19' Bambi had a sweet spot right about 62mph which would hold the tranny in Overdrive at about 1900 RPM's and allow us to ascend most hills of about 2% grade or less. This also seemed to be a good setting when we had the occasional stiff headwind. Obviously on the steeper hills your tranny will need to downshift in order to maintain anything close to a reasonable highway speed and I would recommend locking your tranny out of Overdrive any time it begins hunting while climbing a long grade.

Just my own observations and $.02

-Kevin
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Old 08-05-2008, 04:48 PM   #13
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Our experience..

1995 454 Suburban Throttle body injection, mechanical throttle (cable), got slightly worse (.2-.5 mpg) with cruise engaged. With the old gas prices, not enough of a difference not to use it when safe to do so.

2006 8.1 Suburban Sequential fuel injection, fly-by-wire, just the opposite. Even with the bigger engine and taller gears, 4:10 vs 3:73, the 06 has been averaging 2-4 mpg better than the 95 in or out of cruise.

I'm sure if you were really focused on driving for mpg's you might be able to beat the cruise, but for us the convenience of knowing we're NOT speeding is more than worth it.
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Old 08-05-2008, 06:00 PM   #14
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I have no idea whether our Tundra gets better mileage in or out of cruise because I haven't run tests on the same highway with same weight, etc. Evening out the pedal movement would seem to improve mileage, though on small uphills the cruise shifts the transmission more than I would permit. I'd also let it roll faster downhill, though the cruise will let it anyway. I might give it more gas going down to roll uphill faster like a lot of trucks do. I only use it on 4 lane roads except when there's a lot of traffic and empty 2 lane roads without too many curves. Otherwise, it's too much fiddling with it.

What I like most about cruise is that my right leg feels better after a lot of driving. Thus I feel better. So a feeling better driver is a better driver and nicer husband.

Gene
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