Explain this

[camperforlife]

I have a '11 GMC Sierra 2500 gas. I traded a '09 KZ Spree 261RKS loaded weighed in at 5500 lbs. If I held my speed at 60 I could keep my mileage at 9mpg. Anything over 60 and I would drop to mid 8's.

New trailer is a Keystone Cougar 31RKS that is 7 foot longer and weighs 2300lbs more than the Spree. I just towed 600 miles through the Smokies on I75 up the Jellico pass and averaged 65 mph-70 mph and lowest mpg was 9.3 and tops was 9.8.

The Cougar has a more aerodynamic front with the enclosed propane bottles. Do you think the front end design can make that much difference that would actually increase mileage on that much bigger and heavier trailer?

Thoughts

[DrewE]

Jayco-noslide wrote:
My guess is that going down does not cancel up because the motor is still running and using some fuel?

Modern gas (and I assume diesel) engines do not use fuel when engine braking. It's easy enough with computer controlled injectors to not inject any fuel when the RPM is well above idle and the gas pedal is not depressed at all.

Fuel mileage up and down hills should be a bit lower than on the flat because you generally use some braking (of some sort) on the downhill sides, so the energy you put in going up the hill is wasted in braking going down the hill. If you don't brake but use the momentum to go up the next hill, it's a lot closer to even.

Sometimes the difference between hills and flats is not as marked as one would think because the average speed in the hills is lower, and so the air resistance is lower and less energy overall is required for the drive. Slowing down in flat terrain would often result in better mileage, too, up to a point.

[djgodden]

Don't forget rolling resistance. A higher inflation should reduce this. We switched from all terrain tires to Dura Grappler Highway and have seen an increase of .4 mpg overall. The 2012 2500 Ram CTD we tow with seems to not really care what's behind it. It always gets ~11 mpg.

[Jayco-noslide]

My guess is that going down does not cancel up because the motor is still running and using some fuel?

[Walaby]

Does the up and down hills vs towing flat improve mileage at all? Im just asking because I pretty much do flat towing now, but I know going "downhill", I can see 20 MPG or more.. of course, going uphill has an opposite effect. The one time I did tow in the GA mountains what when I got my best gas mileage with my previous TV and trailer combo.

Mike

[Jayco-noslide]

Fuel consumed is almost always going to be most effected by the weight towed and the speed driven, then hills, wind and to a small degree a few other factors. Also, our usual method of checking MPG isn't really very accurate so MPG could vary on paper from time to time even if everything is unchanged.

[camperforlife]

This is the front design of my trailer quite a bit more aerodynamic that the flat front o the old Spree.

[camperforlife]

failed picture post

[camperforlife]

lbrjet wrote:
You may have had favorable winds (or lack there of) on the trip. I can get 7 or 10 on the same route depending on the wind.

We actually had heavy cross winds until we hit Knoxville then it was more of a tail wind the last stretch from Knoxville to Ashville.

[PawPaw_n_Gram]

Weight impacts fuel mileage most when you are constantly going up and down grades.

Aerodynamics impacts fuel mileage ALL the time.

The 20% increase you describe might be a best case. Looking at pictures/ the design of the two trailers - I'd expect at least 9-12% better fuel mileage.

We joke about getting 15 mpg on a trip to the Rio Grande Valley from Dallas - 7 mpg going down there and 8 mpg coming back. Wind is usually on our nose going down.

I've seen my fuel mileage cut by 30% driving head into a 35 mph wind. Had to be at a specific location at a specific time for a major commitment - otherwise I would have pulled over and waited.

[lbrjet]

You may have had favorable winds (or lack there of) on the trip. I can get 7 or 10 on the same route depending on the wind.

[travelnutz]

Unfortunately, it usually costs a lot more to manufacture towed etc vehicles with rounded corners and/or shaped rooflines which lower the rear flat surface size. Most buyers do not realize it's effect or won't pay the additional cost for having the features and will pay the cost anyway for the extra fuel when driving. When fuel costs are high, they pay the cost difference real fast!

[travelnutz]

Need-A-Vacation,

Yes, what you experienced is factual. As an engineer, frontal aerodynamics is very important as the wind/air resistance and smooth air flow lowers the energy required which increases dramatically as the vehicle/structure's velocity (speed) increases. Actually squares itself! However, that's only part of the equation affecting air/wind energy requirement.

The next highest energy required factor regarding air flow is the suction created at the rear of the vehicle/structure as the parted air displaced by the structure has to refill the space again right behind the moving structure and fill the vacuum that was created by the movement.

This created vacuum sucks backward hard on the moving structure like what occurs in an airplane wing which is sucked up off the ground by the low pressure (vacuum) from the parted air going over the wing shape which pulls the wing upward when trying to refill the vacuum that was created on the top surface of the wing.

The shape and size of the flat rear surface and rounding of the corners on all 4 sides at the rear will make a big difference VS square corners which gives a larger surface in this sucking backward scenario. Rounded corners not only lower the flat surface size but create an easier, faster, and smoother flow for the air to refill the vacuum which lowers the suction forces considerably. Thus, less energy required and less fuel needed. The side and top corners have more to gain than the bottom because air flow is more limited due to it's close distance to the ground surface so it offers the lesser bang for the energy buck but still adds to overall efficiency. Wind tunnel tests have documented the differences over and over for so many decades.

[Need-A-Vacation]

+X for the aerodynamics of the rig....

DrewE wrote:
Aerodynamics do make a big difference in power needed (and hence in fuel milage) at speed, as you've seen. Weight, in itself, is comparatively a non-factor for flat highway driving. For stop-and-go driving at low speeds, things tilt more in the opposite direction.

I don't know if it applies in your case, but the back end of the trailer makes at least as much of a difference in air resistance as the front end. A squared-off back introduces a lot of turbulence and air resistance; this is why practically all new cars have sloping backs, even if it makes for poorer back seat headroom or poorer trunk access or poorer rearward visibility.

I noticed the difference in mpg from the the back wall being shaped differently from our first trailer to our second. First had a slight angle down about 2/3 of the way from the roof line, then actually angled back toward the front of the tt the rest of the way down. Usually saw 10+mpg with the Yellowstone, and was lucky to get 9.5-10mpg with the Dutchmen.

[wa8yxm]

Do I think the areodynamics and thus the design of the trailer (Body shape) can make that much difference...No, I do not think it. I know it can

I also know what a good Wax Job can do for you.. (Recommend it highly) Even somnething like Turtle Wax Zip Waxm, which is kind of "pedesterian" as waxes go, will show you some serious improvement if the trailer has not been waxed in the last 2 years. If it was waxed last week then it won't.