DP Motorhome - Mileage Angst

[swduns]

2008 Tiffin Phaeton 40' DP pulling Chevy Equinox
cummings engine ISC CM2150 8.3L 360HP Gen SN H070089162 8KW
Allison 3000 6spd w/lock up Freightliner custom XC
26,000 odometer miles. Pre-def. No aquahot system
All maintenance performed by Bay Diesel in Red Bay, AL.
I try to drive 55 to 65 depending, never above 65. If I have the time I'll drive 55 or 60. The motorhome is completely stock for the chassis. I do have a Bluefire engine monitor.

I've had the motorhome for several years and just got around to checking the fuel mileage. I've always estimated 8mpg for use in my fueling calculations but it seemed the fuel didn't go as far as I planned.

See attached for my recent fuel mileage. My average is 7.26 which I would interpret as low for a DP. For these trips I drove 65 pretty much.

Of course there are controllable factors: speed, loading, tire pressure, acceleration, etc.
there are uncontrollable factors: elevation change, elevation, etc.

Other than those factors are there any ways to improve mileage.

Thanks in advance.

[saddlesore]

Short version.. Nope or very little..
Use of "Bio-Diesel" will negatively affect Your fuel mileages.

Keep the air, fuel filters and tire pressures maintained as needed, Use the Cruise control..

Keep in mind that You have a 40 footer and that is quite a bit of weight for the small engine to push down the road..
And Your mileage is in line with others of the same class & gross weights..


Look into the "TSD/Open Roads" and the "Mudflap" apps to save a bit on fuel costs..

[n2zon]

We get about the same. I use 7 MPG for planning but 7.5 is about average. We do have an AquaHot but if it's warm it's used less than an hour a day (two showers + dishes), using about the same amount of diesel needed to drive 5-10 miles.

[Ted DuPuis]

In my experience, mileage varies pretty significantly between tanks. It doesn't take much of an uphill or a headwind (or crosswind/tailwind) to impact things. Worst I've gotten was 5.5 MPG heading westbound towards the Black Hills (so uphill the whole way), into a headwind, going 75. Best I've gotten was a bit over 10, eastbound, with a pretty strong tailwind, in the winter. Depending on how your engine is calibrated it may also retard injection timing under certain conditions for emissions purposes (this is even without DEF/DPF/etc. - emissions still existed before that just not in the same ways).

Overall, your numbers look about like what I'd expect. I have improved my efficiency a good bit but it's been a lot of effort to get there. My threads and videos on my YouTube channel show a lot of what I did. My favorite modification for efficiency and smoothness has been the electric cooling fan conversion, but that was also a huge amount of effort.

[Lt Dan]

Over 50,000 miles with my 40' Phaeton the average MPG has been 7.458 so you are not far out of line. Like others, I plan my trips on 7MPG just to be safe because in the mountains it can drop to around 6. I have never averaged above 8-9 except on days with a tailwind and flat travel. I do travel the speed limit up to 70 and didn't notice much difference when we were limited to 55-60 in California.

[Lv2Roam2]

How did you factor in (or out) the generator fuel usage? Should also consider using cruise control when possible with max speed at the power curve (~ 62 - 63 mph) - slowing down to 55+ while remaining in 6th gear is also helpful.

A good 'loaded' weight might tell you something especially if the coach is significantly out of balance and above the 1 HP to 100 lbs total weight. A thousand pounds of fluids (~ 125 gals) and 'stuff' (?? weight) could make a difference.

[darnold9999]

Something that I found made a difference is a constant display of mpg. Will let you know pretty quickly what works and what does not. i.e. I now travel at a pretty consistent 62mph rather than whatever the speed limit is plus 7 - 10 mph. I'm retired so not in a hurry to get anywhere! Stick in the right lane with trucks rather than the left with cars. I needed the scan tool to figure out what my check engine light codes were and leaving it plugged in display's MPG in real time.

Nothing was a huge surprise. Speed beyond roughly 62mpg is the single largest factor. Weight is not really a factor except in mountains and city driving. On the flat wind resistance is everything. The most surprising factor for me was wind. Driving into a head wind pulls mpg down quickly, and a tail wind pushes it back up almost as quickly.

[Skip426]

From your engine manufacturer.

JMHO: When calculating travel related cost in a coach compared to other forms of travel .
Factor in the savings , of having access to your own bed and fridge , three meals out a day and cost difference of an RV site vs a hotel room .

[Flyer15015]

Years ago I learned from an old trucker, to use the "drag-n-fly" method.
Get up to about 62-64MPH and just keep your right foot in the same position.
Don't use the cruise control and gain a bit going downhill and lose a bit going up, but just hold your foot still. It will average out.
Works pretty good in Texas and Kansas. Not so good in Colorado, as the "hills" are too steep and long.

Mike in Colorado

[Rob_M]

Originally Posted by Flyer15015

Years ago I learned from an old trucker, to use the "drag-n-fly" method.
Get up to about 62-64MPH and just keep your right foot in the same position.
Don't use the cruise control and gain a bit going downhill and lose a bit going up, but just hold your foot still. It will average out.
Works pretty good in Texas and Kansas. Not so good in Colorado, as the "hills" are too steep and long.

Mike in Colorado

Yes a good driver can usually get better fuel mileage by using this method vs cruise control. A little run before the hill helps and let it loose a few mph going up. By the time the cruise accelerates going up you’ve lost momentum.

Also turn off the engine brake unless you really need it to stay off the brake pedal. Anything that kills your momentum and shifts you to a lower gear going down hill for no reason hurts your fuel mileage.

I good example is if you are targeting 60 mph in mild rolling hills, let it hit 65 at the bottom of the hill and crest the top of the hill at 55.

[wolfe10]

Yup, for MPG:


Engine brake OFF unless needed.


Allison in ECONOMY MODE.


Cruising speed 60 MPH.


ANTICIPATE SLOWDOWNS AND C O A S T.


You/the driver can make a 20% difference in MPG


OR leave it in "D"/ engine brake on/ power mode and pay the penalty.

[burgmark]

What surprises me is that the mileage for a DP does not seem to vary that much between a large and mid-sized MH. My 35 footer with TOAD weighs 35k lbs. and has the Cummins 6.7L with 360 hp. My best mileage is about 10 and worst about 6, I believe I average about 8. I read somewhere that the 6.7L is the best for mileage and was rated at 12, but presume that is without a TOAD and in perfect conditions. This coming season I'm going to experiment and try to lower my weight to see what kind of impact that has. I tend to pack heavy so I'm sure there are some things I could eliminate.

[wolfe10]

Originally Posted by burgmark

This coming season I'm going to experiment and try to lower my weight to see what kind of impact that has.

If in hilly/mountains it WILL make a difference.


On flat land, very little.


Just straight physics.