Diesel Pusher Gas Mileage

[Scott P.]

A local dealer told me that all Class A diesel pushers get between 6-8 mpg, regardless of their size. What gas mileage do you get (highway driving)?

 

[Lou Schneider]

That's about right. The biggest factor at highway speeds is air resistance and since they are all about the same height and width they have to carve a similar size hole in the air regardless of their length . Best way to get to the high end of that estimate is to slow down, air resistance increases at the square of the speed, i.e. there's 85% more wind resistance at 75 MPH than there is at 55 MPH.

 

[Skookum]

Well, you might get 10mpg with a tailwind and a downhill slope, but your local dealer is pretty much correct.

 

[Larry N.]

Well, while what Lou says about drag (air resistance) is true, and they do have a similar frontal area, the shape is a bit different, and engine and transmission efficiency isn't necessarily the same, especially comparing before and after 2010 built units, so that "all pushers between 6 & 8" is not really true, though it may be OK for general figuring. My 38' 2016 Newmar Ventana (36,400 max gross, 360 HP Cummins) gets typically 8.5 to 10 mpg (rarely 10.5 with strong tailwinds), depending on winds and terrain, and I haven't seen it any less than about 7.5 (with very strong headwinds). Conversely, the 45' 2007 Beaver Patriot (50,000 max gross, 525 HP CAT) I had before seemed to average near 5.5 mpg, occasionally a tad below 5, but rarely above 6.5 mpg.`

Of course technically we get infinite (or zero) gas mileage, since gasoline doesn't figure into it.

All the above figures for both coaches were with normally having the cruise control set on 65 mph, just above where it switches to 6th gear on flat terrain, and pulling the same toad. I've had both coaches over a lot of the same roads at the same time of year (Denver to Quartzsite and back, among others, including Moab and Grand Junction and long stretches of I-70 east of here and other such), so the comparisons are as valid as they can be over a several year period.

Addendum: In response to Skookum, my Ventana seems to run between 8.5 to 9.5 in still air, depending on terrain, with winds and steep terrain accounting for the variations beyond that.

So I'd say 5 mpg to 10+ mpg.

 

[Utclmjmpr]

I agree with Larry,, altho I get a flat 10 with my pre electronic 8.3 Cummins My 25 year old coach is also a bit more aerodynamic than later forms that are more squarish in the front and corners.. My speeds are normally 67 MPH to keep the old girl in 6th gear towing a Jeep.. Weight at 26750 all up and ready to travel with the Jeep putting me up to 31K..>>>Dan ( Also a "diesel pusher" does not get ANY "gas" mileage..)

 

[JD Sharp]

Our last motorhome was 38' Tiffin with 340hp Cummins, we got up to 10mpg on long snowbird trips. It's all about weight, Tiffin weighed 24,000 lbs and 2004 Beaver before the Tiffin was 33,000 lbs

 

[Kevin Means]

Our 43 foot coach weighs in at roughly 43,000 lbs. (without toad) and about 48,000 lbs. when towing. Over the last nine years and roughly 55,000 miles, we've averaged 7.4 mph when not towing, and 7.2 mpg when towing. Most of our driving is out west, often in the mountains, and I always use the cruise control set at 65mph.

I think that's pretty decent mileage considering the size and weight of both vehicles.

Kev

 

[UTTransplant]

It really does vary with your weight and engine, but not a bunch. We are at 32,000 pounds flat towing a 5,300 pound Grand Cherokee. 6.7L 360HP diesel. The average reported by the chassis/engine data bus is 8.5 mpg over 60,000 miles. The majority of that is in the mountain west, though we do come home to the Midwest occasionally! I drive at 62-64 mph because that is the sweet spot for my transmission/engine combo, but I go along with traffic speed in urban areas which is more like 65-68 mph.

 

[Scott P.]

Thanks everyone. I appreciate all of your responses.

 

[CityGuy]

A local dealer told me that all Class A diesel pushers get between 6-8 mpg, regardless of their size. What gas mileage do you get (highway driving)?

I get 8 rolling on flat ground, no wind, at 55; about 7.5 at my 63-64 sweet spot. Average, 6.8 to 7.2. 33,000 lb 36 foot class A. 2006, so not the latest technology. I keep a log of genset use and subtract 1.5 gallons of diesel per hour of its use when computing an average. Dash air or gen-powered coach air seems to reduce the average about the same.

 

[Gary RV_Wizard]

My 40 ft diesel ran around 6.5-7.3 on two lane roads and 8.0-8.5 cruising the interstate at a steady speed of about 64 mph. A 2002 gas-powered 36 footer delivered a pretty consist 8.0 mpg on the highways and around 7 in towns. A 1996 35 footer struggled to get up to 8 and only managed in ideal conditions - it was mostly around 7. The big diesel weighed just about exactly twice the old 35 footer, so weight isn't the determining factor at all.

In 2007-2009 the bigger diesels suffered badly from newly added emissions controls and mpg dropped to 4-6 mpg on many 40-45 footers, but it recovered some starting in 2010 as technology improved.

 

[Larry N.]

The big diesel weighed just about exactly twice the old 35 footer, so weight isn't the determining factor at all.

True for cruising down the Interstates, and true on much of the state & U.S. highways, though those often go through towns, but in town for any distance there's often a lot of start & stop, where weight does make a big difference.

In 2007-2009 the bigger diesels suffered badly from newly added emissions controls and mpg dropped to 4-6 mpg on many 40-45 footers, but it recovered some starting in 2010 as technology improved.

Yes- my 2016 Ventana rarely runs below 8.5 mpg, sometimes a little over 10.

 

[Ray-IN]

I think that is a good estimate, my MH weighs 31,000#, has a Cummins ISC8.3 and averages 7 mpg, and I never drive to maximize mileage.
Final cost and fuel mileage is the main reason MH mfgrs. began using the Cummins ISB6.7 medium-duty diesel engine instead of the ISL8.9 heavy-duty and larger engines.
There is a set amount of BTU's in one gallon of #2 diesel fuel, the smaller engine uses less fuel, resulting in higher mileage, and produces less HP as a result.
For instance the Cummins 15L engine averages 4-6 mpg in a semi-power unit moving 80,000#.

 

[Utclmjmpr]

The ISX at 600 HP does better than that at the same weight.. And a whole lot better on hills.>>>Dan

 

[JD Sharp]

Sorry Gary, but here are stats on loaded and unloaded milage
Equipment: 2001 International 9400i condo, 12.7L Detroit, 10 speed direct, 2.64 rears, 6x4, pulling a mildly ribbed dry van:

Bobtail (18,250 lb):
55mph: 12.5-14mpg

Empty (32,500 lb):
55mph 9.5-11mpg
65mph 8-9.5mpg

Fully Loaded (80,000 lb):
55mph 6.5-8.5mpg
65mph 5.5-7.5mpg

The range is because there is that much variance depending on terrain & wind.

 

[Lou Schneider]

Sorry Gary, but here are stats on loaded and unloaded milage
Equipment: 2001 International 9400i condo, 12.7L Detroit, 10 speed direct, 2.64 rears, 6x4, pulling a mildly ribbed dry van:

Bobtail (18,250 lb):
55mph: 12.5-14mpg

Empty (32,500 lb):
55mph 9.5-11mpg
65mph 8-9.5mpg

Fully Loaded (80,000 lb):
55mph 6.5-8.5mpg
65mph 5.5-7.5mpg

The range is because there is that much variance depending on terrain & wind.

The difference is the 10 speed transmission in the International tractor versus the 4 or 6 speed automatic in most motorhomes. More gears lets the engine stay closer to it's most efficient speed (the sweet spot).

 

[A Traveler]

Cummins 400 hp ISL in a 2006 Mandalay. 32,000 pounds.

Over 132,000 miles it has averaged 7.3 miles per gallon.

 

[Skookum]

I think that is a good estimate, my MH weighs 31,000#, has a Cummins ISC8.3 and averages 7 mpg, and I never drive to maximize mileage.
Final cost and fuel mileage is the main reason MH mfgrs. began using the Cummins ISB6.7 medium-duty diesel engine instead of the ISL8.9 heavy-duty and larger engines.
There is a set amount of BTU's in one gallon of #2 diesel fuel, the smaller engine uses less fuel, resulting in higher mileage, and produces less HP as a result.
For instance the Cummins 15L engine averages 4-6 mpg in a semi-power unit moving 80,000#.

So we’re talking pushers but I’m not sure that adds up, the part about 6.7 vs 8.9. Maybe it is cost alone.

A tale of two Super C’s…
The 6.7 (2021 version) we had has the same horsepower as our 8.9 (360), also revised for 2021.

They get roughly the same mpg, between 8 and 9, but the L9 puts out 350 lb-ft more torque than the B6.7 in this configuration and the rig weighs significantly more, about 38k loaded. Gross rating on our Mh with the 6.7 was only 31k, and it scaled, travel ready, around 25k.

Both MH’s have roughly the same profile, the Super Star is a little taller, both roughly the same length (40). Different transmissions but similar gearing and axle.

To me, it seems like these engines all suck about the same amount of fuel in a motorhome until a certain weight where a bigger turbo and more gearing is needed.

 

[Ray-IN]

Skookum, HP to weight ratio largely determines fuel consumption rate, but by far the biggest factor is the driver.

 

[Skookum]

Skookum, HP to weight ratio largely determines fuel consumption rate, but by far the biggest factor is the driver.

That's where I'm not so sure...with DP's and large-profile motorhomes, I think wind resistance is as big a factor as driving style. Driven on the same terrain by the same conservative driver, a 6.7 or 8.9 will probably get about the same mileage in two motorhomes of different mass, provided they have about the same profile for wind resistance.

Maybe I'm off my rocker here but I was sure we'd take a huge hit in fuel mileage when we went from a 6.7 to an 8.9 with an increase in weight too, but the difference appears to be almost negligible. I tend to be a conservative driver. Maybe the 8.9 would suffer much more in town, but I'd estimate 95% of our miles are highway.