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Old 05-13-2012, 10:46 PM   #1
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7.9 MPG towing - reasonable?

Just returned from our first long trip. We towed for 1,500 miles on mostly interstate highway and (according to onboard data) we averaged 7.9 MPG for the entire trip. Actually, I was expecting less than 7 mpg, so I was pleasantly surprised. On some legs of the journey we experienced 8.4mpg, other legs we got 6.8 mpg.

I kept the cruise set around 62mph which helps fuel milage, but more important, I find this to be a comfortable driving speed and I don't have to pass very often.

What's your experience with towing milage on interstates or open road?

cheers,
Joopy

PS - C-7 diesel
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Old 05-13-2012, 11:23 PM   #2
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I have the same engine in an 07 Expedition 38V and tow an Olds Alero. Run about same speed and approx 1750 rpm for a 7.8 to 8.0 mpg. At 55mph and approx 1600 rpm or so the mileage is up to approx 8.5 to 8.8 mpg.
There are lots of variables as you are probably aware...water in tanks, fuel load, winds, etc , etc......
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Old 05-14-2012, 05:24 AM   #3
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I have 2004 Journey 39K with a cummins 330 and drive around 65 and avg 7.8-8.0. I do not do as well around 55 don't know why.
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Old 05-14-2012, 06:32 AM   #4
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Quote:
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I have 2004 Journey 39K with a cummins 330 and drive around 65 and avg 7.8-8.0. I do not do as well around 55 don't know why.
Our motorhome is gas, so I can't speak from first hand experience, but I'd guess the reason your mileage is less at 55 is because the engine rpm's are a little lower and engine isn't producing at much horsepower at that particular rpm. Difference from 55 to 65 isn't causing that much more resistance with aerodynamics and the engine is working easier.
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Old 05-14-2012, 07:52 AM   #5
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VERY reasonable. That's exactly what we get with 8.3 Cummins @ 330hp with full Banks. We pull a Saturn Vue @ 63-65mph.......
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Old 05-14-2012, 07:55 AM   #6
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Just got back so this is pretty fresh in my mind. 5500 miles, 6 1/2 weeks, S.E. Mi to Florida Panhandle, then following along the Gulf Coast to Corpus Christi, across Texas (and some pretty good sized hills), New Mexico, then Arizona to Phoenix, north through Flagstaff (more big hills!), Moab Utah, then on up to Steamboat Springs, Co and home again. We run X-ways where possible, and nearly always 65mph.

We averaged 8.4mpg with a high on one leg of 9.6, and a low on another of 7.8 (stiff headwinds, lots of hills).

Edit: should mention this included about 50 hours use on our generator when we were going through some of the southern states in 100 degree heat!
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Old 05-14-2012, 08:02 AM   #7
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I have 2004 Journey 39K with a cummins 330 and drive around 65 and avg 7.8-8.0. I do not do as well around 55 don't know why.
If you look at an engine's torque and BSFC (brake specific fuel consumption) curves, you'll find that best BSFC normally coincides with peak torque RPM. This is the engine's "sweet spot" where it also produces the highest BMEP (brake mean effective pressure). I would bet that you're closer to the torque peak RPM at 65 than you are at 55.

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Old 05-14-2012, 08:54 AM   #8
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That's exactly what I got on mine this weekend.
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Old 05-14-2012, 09:17 AM   #9
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Ford Triton V-10 ave. 7.5 mpg dragging Corolla on a dolly about same average speed...62 mph or so for 450 miles back and forth through L.A. and Grapevine.
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Old 05-14-2012, 05:18 PM   #10
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I would be happy with that. My Freightliner dash trip computer has a non-resetable fuel mileage readout. 7.2 mpg for 32,000 miles since new. C-7 350.

Jim
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Old 05-14-2012, 06:42 PM   #11
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The answer to your question depends on many things. How fast you drove, what you are driving, and what you are towing are among the top questions.

That said.. 7.9 is well within the "Normal" range for 30-40 foot RV's
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Old 05-14-2012, 06:53 PM   #12
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My MH weighs out about 225000 and I tow a Lexus RX 300(5000 lbs) Don't quite have the power that you guys have and I average 10-12. I think this is exceptional mileage. I have a very light foot and never in a hurry. I also contribute the good mileage to my MD3060 tranny. This is is on straight and level roads. For the last 80,000 with multiple owners the mileage in the Cummins computer is about 10.5
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Old 05-14-2012, 07:23 PM   #13
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Quote:
Originally Posted by slick60
I have 2004 Journey 39K with a cummins 330 and drive around 65 and avg 7.8-8.0. I do not do as well around 55 don't know why.
At 55 I am still in fifth gear I don't hit final drive until around 61 or 62. That could be one reason for decreased fuel mileage.
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Old 05-14-2012, 07:33 PM   #14
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At 55 I am still in fifth gear I don't hit final drive until around 61 or 62. That could be one reason for decreased fuel mileage.
Sounds like some of those big motorhomes need the rear ends regeared (Or a two speed rear end) so they get better fuel economy at slower speeds .
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