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Old 01-07-2016, 01:29 PM   #15
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I notice a big difference between 65 MPH and 70Mph
So true. In addition, you may want to lower your expectations of how fast you go up a hill. I've decided that if I'm catching the semi's, I'm doing just fine. If it's a race, you're in the wrong vehicle.
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Old 01-07-2016, 01:48 PM   #16
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I look at it this way: it's a motorhome. You'd be hard pressed to make it aerodynamic andif you did, you wouldn't have a motorhome. And I don't care about MPG, I care about CPM, cents per mile, and the comfort of the coach given my lifestyle. I get 8mpg and spend 2.50 per gallon, that's 32 CPM. Why would I sell this one and buy something new to save money when I'll take at least a $50k hit on the transaction. The difference between 8 and 10 MPG at $2.50 per gallon is 7 cents per mile. That $50k loss is gonna take over 700,000 miles to recover at 7 cpm savings.


I'll keep SOKI, she works just fine. (Spending Our Kids Inheritance)
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Old 01-07-2016, 02:37 PM   #17
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"and the 330 avg about 10 mpg"

Well Sir,
If you've experienced that kind of AVERAGE with a 330, I must have gotten the bottom end of the production C-7 330 units. I drive, and have always driven very mildly and keep that big beast at or around 62 mph while out on the open road. Even when we're not towing anything, which is seriously rare, I'm really, really lucky if we get at or close to 8. I've got some buddies that also have the 330 and each and every one get almost exactly what I do. Hmmmm.
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Old 01-07-2016, 03:41 PM   #18
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Just as an aside, I never bother calculating mileage anymore. I let the computer on the dash give me a number (usually 10.1) on the Excursion.i know that number is not real and does not account for generator usage, but it is consistent. I can see it vary when I change cruising speeds over long periods of time. If the computer starts giving me a number drastically lower than that, I'd have to start wondering what is going on with my vehicle.
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Old 01-07-2016, 03:54 PM   #19
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RV and MPG

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Old 01-07-2016, 04:06 PM   #20
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I use 7.5 for figuring. Our DSDP with 350 ISC (then added a Banks for 435/1200) would do between 7.5 and 8 overall over 48,000 miles.
With the present rig the VMS shows an overall of 7.6 for 45,000 miles. As to slowing down on the hills, I haven't noticed it at all. Go up the Grapevine on cruise control at 55 with no drop in speed and I can easily recover if I get slowed down by traffic. However, there aren't many 650 hp rigs out there either. Our empty weight is 46,000#'s so I figure 7.6 is good enough, especially since I still get that mileage loaded and towing a Honda Odyssey.
At 60 mph I'm showing 1350 on the VMS and tach. Rear gearing is 4.3 with the 315/80 drive tires. The calculated "sweet spot" for mileage is 1375 according to Cummins.
I wasn't worried about the fuel mileage, if I was I'd be driving a Prius and staying in motels.
Mr_D, 7.6 in that monster ISX and coach is phenomenal!

I've never calculated the actual but the display hovers around 7.5 or so. I just have never worried about MPG in any of my vehicles. maybe I should.
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Old 01-07-2016, 04:15 PM   #21
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As stated by several, it is just physics: weight and aerodynamics. A larger coach will also weigh more. As the example by twinboat, no matter what the max hp rating of your engine, it will make what is required to move down the road. Up the hill takes more power, more power equals more fuel used.

I figure mpg is what it is, enjoy the trip and just accept the fuel bill as part of the total cost.
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Old 01-07-2016, 09:56 PM   #22
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fuel milage

I sold HD diesel trucks for many years. Customer Joe would come in and say that customer Bills truck is just like mine and he says he gets 9MPG.
and I only get 7MP. How can I get 9MPG. My answer was ,do what Bill does, Lie.
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Old 01-07-2016, 10:08 PM   #23
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While driving a 500 hp 45ft Signature, I did appreciate the power that comes with that big hp motor. But while I was able to go up hills pretty nicely, I did notice that the MPG was running a tight 2 mpg average! I had to slow down a lot to get it up to almost 6 mpg. I did see some 6 to 8 mpg going downhill! Even on a straight and level stretch, if I did not watch the speed, I was really eating fuel at the 2 to 3 mpg average. Just sayin.......
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Old 01-07-2016, 10:10 PM   #24
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Mr_D, 7.6 in that monster ISX and coach is phenomenal!
Actually that seems to be about what other owners with the ISX 600 or 650 get in similar rigs. Since the VMS goes by what Cummins electronics reports it doesn't change due to usage by the AquaHot.
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Old 01-07-2016, 10:13 PM   #25
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While driving a 500 hp 45ft Signature, I did appreciate the power that comes with that big hp motor. But while I was able to go up hills pretty nicely, I did notice that the MPG was running a tight 2 mpg average! I had to slow down a lot to get it up to almost 6 mpg. I did see some 6 to 8 mpg going downhill! Even on a straight and level stretch, if I did not watch the speed, I was really eating fuel at the 2 to 3 mpg average. Just sayin.......
On our rig it does go down going up a hill, but it goes way up on the way down, in fact I've seen 256 mpg going down. On the flats I see 13 to 15 mpg on the VMS so it averages out.
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Old 01-07-2016, 10:57 PM   #26
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Our 08 40ft had a cat c9 at 400hp and it seemed about right for hp and got around 6 mpg. We went to a 08 45ft with a cummins at 600 hp and got a little better mileage My observations are the 600hp feels similar to driving a pickup as far as hills and get up and go . It would seem the newer engines may be getting better mpg which goes against my previous thoughts that it was all about weight. On the last trip my wife drove a few hours and commented how she was surprised that once cruise was set there was no change in speed going up or down rolling hills. Another note you get the added braking provided by cubic inchs with the jake brakes. Some people don't care about the hp and that's fine to.
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Old 01-07-2016, 11:05 PM   #27
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We have a 45ft Monaco Signature with a 14 liter Series 60 Detroit diesel and pull an 09 Ford Explorer. We will typically average 6-6.3mpg with the cruise control set on 65mph. Mileage drops to 4.9-5.5mpg at 75mph. We're grossing approximately 51,000 pounds. We have broken 7mpg with the cruise control set at 58mph. Unfortunately I'm not retired so we rarely have the time to drive that slow.
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Old 01-07-2016, 11:10 PM   #28
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Our 08 40ft had a cat c9 at 400hp and it seemed about right for hp and got around 6 mpg. We went to a 08 45ft with a cummins at 600 hp and got a little better mileage. My observations are the 600hp feels similar to driving a pickup as far as hills and get up and go. It would seem the newer engines may be getting better mpg which goes against my previous thoughts that it was all about weight. On the last trip my wife drove a few hours and commented how she was surprised that once cruise was set there was no change in speed going up or down rolling hills. Another note you get the added braking provided by cubic inchs with the jake brakes. Some people don't care about the hp and that's fine to.
The three position engine brake (ours uses the variable vane turbo in conjunction with the jake) is extremely strong when on high and I very seldom use that setting.
Getting onto a freeway is another experience too. I've had truckers pull over to give me room and I usually have to feather the throttle to let them back over.
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