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Old 06-18-2011, 12:15 AM   #15
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Originally Posted by Diplomat Don View Post
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.
Just my two cents!!!! Based on our 360 HP ISC performance, all you folks are extremely fortunate. We check our fuel mileage religiously and over the 30,000 miles since purchase new, our best has been 8.3 mpg with a huge tail wind all day long. Have not checked cumulative average but bet that it is not much more than 7.3 or 7.4 mpg, and speed does not seem to influence it much. We always start the trip at 58-60 mph and soon find ourselves at 66-68. We experience more impact on mpg from wind rather than speed of travel. I have been told it will improve as we get more miles on the rig, but am not holding much hope.
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Old 06-18-2011, 01:06 AM   #16
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Last time out I was "playing" with my VMSpc and it seemed to indicate that I got better mileage at about 67-68 than at 60 by a couple mpg. Need to do more testing on longer trips to be sure. I was using my GPS to check the speed and the VMSpc to monitor the mileage.
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Old 06-18-2011, 01:15 AM   #17
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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.
According to some experts the best mileage should be about 200 rpm over torque peak. But on our ISC with Banks my torque "peak" is from 1400 to past 1700 rpm. If I tried to run at 1900 rpm I'd be collecting tickets every time out. I do seem to get better mileage at 1700 rpm than 1500 though. Stock torque peak is at 1400 rpm. Seems the Banks kit changes things a bunch.
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Old 06-18-2011, 08:13 AM   #18
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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.
My DP with the Allison MH3060 transmission.
Will go into 6th gear at 52 MPH every time.
In MODE setting or not.
Probably around 1,600 RPM or a little less after the shift.

And then will run in 6th until the speed drops below 48 MPG.
Unless the pedal is pushed to hard to make it downshift back into 5th.

For the ones that can't shift into 6th until much later(62-67) and the engine will pull without stress at 55 MPH. Or be within the engine torque range.

You do know any Allison shop can program yours to shift earlier.
It may give you payback in MPG in a short time to pay for the programing.

When driving along at my 55-58 MPG speed. I have noticed those that need to drive at higher speed to stay in 6th. Pass me 2 or 3 times during the same day.
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Old 06-18-2011, 08:20 AM   #19
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Engine RPM is a big factor in fuel economy. Assuming that the engine isn't accelerating or working too hard, the engine will have a given fuel to air ratio. An engine is basically a large air pump. The faster it turns, the more air (and fuel) it pumps through it. By keeping in a high gear you keep your RPM as low as possible. Of course driving too slow gets you out of the power band, which necessitates a dowshift to rev the engine up so you lose if you go too far with that analogy.

But the biggest load upon the RV is not rolling resistanace, pulling the toad, or overall weight. It's wind resistance. At 55 MPH the wind resistance on a typical class A is around 320,000 lbs. At 70 MPH that goes to 520,000 lbs and at 80 MPH it jumps up to 680,000 lbs. It takes horsepower to move that much air (weight) out of the way and now your fuekl economy takes a double hit because your engine RPM went up at the higher speeds and your fuel-air ratio probably increased if the engine needed to work harder.

That's one of the reasons why the guys with the 500 HP Spartan K2 chassisd get as good or even better mileage than some of the 400 HP ISL drivers. The 500 HP ISM has bigger cylinders but the rear axle gear ratio is much better so the engine just lopes along at lower RPM. Plus, that 500 HP ISM doesn't need to break a sweat and increase the fuel-air ratio very often.

More info on RV Tech Library - Fuel Economy Tips
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Old 06-18-2011, 12:48 PM   #20
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Originally Posted by Diplomat Don View Post
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.
Science also says that helicopters can't fly and that concrete won't float. In the real life world, we know that things aren't always what they seem.

I've tried to make the transmission shift into 6th at the lower speed, but, it doesn't work that way. And, when it is in 6th and the speed drops below 61 or so, either by grade or backing off the throttle (we have a few small grades on the west coast) it automatically goes back to 5th and won't upshift until 63 mph again. In 5th gear the mileage is 1 mpg lower than 6th. I've tried to explain the science to the fuel pump, but it doesn't seem to listen. I just hate it when reality interferes with book learning.

Since we have motorhomes built by different manufacturers, with different engines, different transmissions, different loaded weights, and different driving locations, I would bet that we all have different experiences with our mileage. I enjoy hearing the different experiences and comparing them with mine, even if they don't agree with science.
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Old 06-18-2011, 02:41 PM   #21
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I do seem to get better mileage at 1700 rpm than 1500
Same with my 350hp ISC ...I believe one reason is that on grades the automatic downshift from 6th to 5th happens at higher RPM, keeping the engine at higher rpm and decreasing the likelihood/need of a further downshift to 4th in many cases.
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Old 06-18-2011, 03:04 PM   #22
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bucks2....Read the posts more carefully before you make your comments. I wasn't refering to you with the 10 mph comment. Obviously, common sense says there will always be factors that affect the results. 35 mph vs 45 mph will probably not make much of a difference in wind resistance, but it sure changes when you're getting in the 55 to 65 range as another stated.

I didn't tell anyone how to shift their coach but offered a method to get it back into 6th. I'm sure your 43' Beaver is very heavy and would not want to shift into at a lower speed, so don't use my suggestion.
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Old 06-18-2011, 03:29 PM   #23
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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
Using the Mode button may actually be hurting your mileage on your RED. Over the last 5500 miles towing our HHR 95% of the time, we are at 9.9 mph on the computer. It was consistently over 10mpg when we were in Texas, NM and Arizona.
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Old 06-19-2011, 09:05 AM   #24
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Originally Posted by Mr_D View Post
Last time out I was "playing" with my VMSpc and it seemed to indicate that I got better mileage at about 67-68 than at 60 by a couple mpg. Need to do more testing on longer trips to be sure. I was using my GPS to check the speed and the VMSpc to monitor the mileage.
The VMSpc will also give you speed.
I have adjusted mine to match the GPS.
Mine sits on the dash and is easier to see then the MH Speedometer.
And the MPG & MPH are close to each other. No need to take eyes off Laptop and look another place for speed.
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Old 06-19-2011, 10:07 AM   #25
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I've noticed the grooves on the concrete road surface has an affect on economy. The parallel groves seem to be so much better than the perpendicular. I found this interesting article on fuel economy.
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Old 06-19-2011, 08:13 PM   #26
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bucks2....Read the posts more carefully before you make your comments. I wasn't refering to you with the 10 mph comment. Obviously, common sense says there will always be factors that affect the results. 35 mph vs 45 mph will probably not make much of a difference in wind resistance, but it sure changes when you're getting in the 55 to 65 range as another stated.

I didn't tell anyone how to shift their coach but offered a method to get it back into 6th. I'm sure your 43' Beaver is very heavy and would not want to shift into at a lower speed, so don't use my suggestion.
I'm not sure what you're angry about. I quoted an entire paragraph out of your post to make sure the context was accurate. I then stated that my coach actually got better mileage at the higher speed, which appears to be the opposite of what I think you're trying to say, but not saying clearly. (are you saying mileage will go up or down at the 55-65 range, you haven't said other than "it sure changes".)

Your post identifies a possible solution. I stated that your solution did not work on my coach.

So which part am I not reading carefully enough for YOUR satisfaction?

Ken
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Old 06-19-2011, 08:46 PM   #27
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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?
Yes, slower = higher MPG. However, I think your units of time may need to be recalibrated. For a trip that long, I'll measure time in days - how many days travel do I have before me? We do Chicago -> central FL several times a year, and it is 2 overnights en-route, regardless of how fast we go. We have favorite stops along the way (traveling w/ young children, I'm not game for an experiment in finding an RV park at the end of the day), so even if we keep the hammer down the whole trip, it's meaningless: get to FL at noon instead of 1pm on the last day.

Conceivably, faster could be an extra fuel stop. That's 20 mins.

So for the long trips, we find acceptable overnight spots and count days, not hours. YMMV.
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Old 06-20-2011, 06:33 AM   #28
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Yes, slower = higher MPG. However, I think your units of time may need to be recalibrated. For a trip that long, I'll measure time in days - how many days travel do I have before me? We do Chicago -> central FL several times a year, and it is 2 overnights en-route, regardless of how fast we go. We have favorite stops along the way (traveling w/ young children, I'm not game for an experiment in finding an RV park at the end of the day), so even if we keep the hammer down the whole trip, it's meaningless: get to FL at noon instead of 1pm on the last day.

Conceivably, faster could be an extra fuel stop. That's 20 mins.

So for the long trips, we find acceptable overnight spots and count days, not hours. YMMV.
We too try to time it in days and find our usual layover spots for rest. Honestly the post was as much for conversation as information since we rarely worry about mileage. It gets what it gets and we cringe but fill up when it gets low. Wouldn't bother me to get better economy of course.

We have made it all the way from central NY to the Keys in 2 or 2 1/2 days by keeping it at (or maybe just above) the speed limit and only stopping for fuel and the minimum amount of sleep. Both of us driving too. Now that we have retired there is no rush. Last fall/winter the trip that used to take 2 days took 2 months We have a friend that tows his 29' boat down and drives 20 hours, sleeps for a few and gets right back on the road by himself. How he does it I do not know.
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