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Old 06-17-2011, 07:03 AM   #1
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Slower for fuel economy?

I understand that with a DP you can greatly improve fuel eco by going 55 instead of 65. Just for fun using a this Time, Speed and Distance Calculator so I don't have to do the math I plugged in the approximate distance we travel each winter to escape the snow. 1600 miles at 55 takes 29 hours 5 min. At 65 24 hours 37 min. So round trip losing about 4 1/2 hours each way we lose an entire day, is it worth it? We try not to be in a hurry but when going from cold to warm it it is nice to get there. On the way back, who cares.

I understand it is not possible to maintain a constant 55 or 65. I also will not go slower anywhere that I might aggravate other drivers. I don't like being behind someone going to slow so I won't put others through it. It also can make people make bad and dangerous decisions on passing.

So has anyone tried this and had enough fuel economy improvement to take the extra time?
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Old 06-17-2011, 07:17 AM   #2
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My DP gets the same mpg at 55 or 65...I guess if you could drive a constant 55 MPH
never slowing or accelerating in theory you would get better mpg. IMO
Peace...D
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Old 06-17-2011, 07:44 AM   #3
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Just did a 700 mile round trip as a test and got 9.3 running at 70 mph, cruise control on much as possible. This is acceptable mileage for me, so I plan on running this as a normal speed. A diesel should get its best fuel mileage at torque peak in its highest gear, which would be 56mph for me. I'd have to take NoDoze.
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Old 06-17-2011, 07:45 AM   #4
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I tried the 55 mph on a trip last year. My fuel economy appeared to be better. I say appeared, because I did not make the same trip doing my normal 60-62 mph. I'm not sure that I'm going to repeat the 55 mph's again since on this trip from Texas to Mass. I obtained 10.6, 9.6, and 8.9 on three different segments of trip, depending on terrain. So, I'm not going to complain.

Considering that I typically travel about 250 miles a day and enjoy the scenery, it is only 12 minutes at at 60 vs. 55 mph.

Thanks for the link. I'm bookmarking it.

My math is that if 2 + 2 = 5, then 2 + 2 = 4. That being a truism since if anything + anything = anything, then 2 + 2 = 4. Some times I get confused.
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Old 06-17-2011, 06:17 PM   #5
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I just can't drive 55.
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Old 06-17-2011, 06:50 PM   #6
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I get better MPG at 55-58 MPH then at 60-63 MPH.
I don't ever remember of gaining a day driving 65-68.

Retired and a 250-350 mile day works for me. From campground to campground.

55-58 MPH average MPG 10.4 to 10.7
60-63 MPH average MPG 10.1 to 10.2
65-68 MPH average MPG 9.7 to 9.9
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Old 06-17-2011, 06:52 PM   #7
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I deisel pusher will get the best fuel economy at the slowest speed it will stay in 6th gear. For mine, that is 62. At 55, I an in 5th gear, and turning a higher RPM.
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Old 06-17-2011, 07:04 PM   #8
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Our DP does not even go into 6th gear until about 67 mph.
5th & 6th gear are both overdrive and we get better fuel mileage running about 62-65 mph in 5th than 55 mph in fourth gear.
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Old 06-17-2011, 08:13 PM   #9
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Quote:
Originally Posted by Coma View Post
Just did a 700 mile round trip as a test and got 9.3 running at 70 mph, cruise control on much as possible. This is acceptable mileage for me, so I plan on running this as a normal speed. A diesel should get its best fuel mileage at torque peak in its highest gear, which would be 56mph for me. I'd have to take NoDoze.
Holy-e -Moses, 9.3 mpg at 70 mph? Are you sure you did your math correctly? I can't get that kind of milage at 55 mph.

Jim E
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Old 06-17-2011, 08:28 PM   #10
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On my gasser the "Knee" is right around 60 mph,, Below 50 slowing down may actually COST MPG as the tranny shifts to a lower gear.. Over 60 the averge MPG drops faster than the MPH increases.. So 55-60 is the "Sweet Spot" Unless, of course, you are going down hill with a 100mph tail wind (Been there too)
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Old 06-17-2011, 08:31 PM   #11
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Originally Posted by 38 Special View Post
I deisel pusher will get the best fuel economy at the slowest speed it will stay in 6th gear. For mine, that is 62. At 55, I an in 5th gear, and turning a higher RPM.
There we go! I tried the 55-58 mph and got horrible mileage. At 63 with the "mode" button activated, it stays in 6th and the mileage improves almost a mpg. Which makes me hate driving in CA and trying to stay close to the legal speed of 55 and then the double penalty of paying the higher price for fuel.

Ken
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Old 06-17-2011, 09:37 PM   #12
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I drive a 40' MH powered by a C12 Cat towing a Ford Explorer. On my last trip from AZ to WA and back to AZ, a little over 3200 miles, I averaged 7.06 mpg traveling at 60 mph on cruise control. I am currently making the same trip at 55 mph. After 2276 miles, my average mpg is 8.6. If my math is correct, that is $320 difference over a 3,000 mile trip at $4 per gallon.

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Old 06-17-2011, 10:30 PM   #13
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Originally Posted by okgc View Post
Our DP does not even go into 6th gear until about 67 mph.
5th & 6th gear are both overdrive and we get better fuel mileage running about 62-65 mph in 5th than 55 mph in fourth gear.
I think you need to push the Mode button. My 2010 36 ft Red will get into 6th easily at around 62 mph. I have even noticed it getting into 6th at 58 mph in long construction zones.

mpg is over 9 in flat land and under 9 out west
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Old 06-17-2011, 11:28 PM   #14
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We recently took a trip to the Grand Canyon and decided to try 57/58 instead of our usual 63. We've been averaging 8.8 for 40000 miles. On the first two thirds of the trip we got 10.2 with easy throttle ups and slow downs. On the last third we hit some pretty good winds and dropped to 9.6. We ended up with an average of 10.0 for the trip. We were surprised to see that most of the trucks were keeping pace with us.

I have to question anyone who says there mileage doesn't change when they change speeds by 10 mph. Science/wind resistance would disagree. For those who say they can't go that slow because there coach won't go into 6th, try accelerating until it shifts and then back off the throttle. It will usually stay in 6th until you hit a grade......or just say you're not willing to go that slow.

Finally, "HD4Mark", you're math is accurate, but I doubt you're doing the drive in one day. Most would do that trip in three days which only adds about 90 minutes to each days drive.
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