Surprising Fuel Savings at 55 mph

[Crawdad]

I really like the idea of the air speed indicator also. Where can I find one and where is it installed?

[slowmover]

AIRCRAFT SPRUCE for air speed indicators. Where to mount the pitot tube is the real question.

A 20% decrease in fuel burn for a 15% decrease in travel speed is a great return. And Crusty nailed it with his explanation. The other piece of the puzzle is that rolling resistance is the greatest forward impediment up to 45-mph. So, 55 is really kind of a sweet spot.

Don't forget that wear & tear on the rest of the vehicle is greatly reduced. Tires last a good deal longer, for one, and drivetrain components live an easier life as well.

Reaction times are all better, and coming to a complete stop in a controlled manner is much more likely.


The basics of FE apply, though:

1] Perfect axle alignment, TV & TT
2] Zero brake drag (both vehicles) and bearing adjustment (TT)
3] Proper tire choice and perfected tire pressure (both vehicles)
4] Zero steering slop
5] All book maintenance on TV
6] WDH set up on certified scale

The money is in spec'ng the TT & TV for economy first. The rest is in:

A] Climate
B] Terrain
C] Vehicle use (read as road type, speed, etc)

An A/S is the best choice for FE, but the TV can return high numbers if chosen well.

On my truck I don't drop below 24-mpg when solo at 58-mph (the fastest high mpg speed), and towing is above 16-mpg. All this on level Interstate, low altitude, warm temps, etc . . . favorable conditions. But it is also with cruise control and air-conditioning, rain or shine, heavy traffic or not.

My 20% loss comes when I bump it all the way up to 65/66 and it falls off to 12 +/-.

Gearing and aerodynamics are -- past driver skill -- about the only way to increase mpg for a given combination at a given speed. Going "slow" is just the start . . but accounts for the biggest change.

.

[A W Warn]

In the last 3 weeks I towed about 1,500 miles on a trip of 2,000 miles total travel. On the first leg of the journey I got a late start so I was in a hurry to meet friends on time, I was running 60-65 mph. Afterward I slowed down to my normal speed of 55-60 mph. I was supprised to see just over 10% increased efficiency (measured, not computer) in mileage when running at the slower speed, comparing those different days.
The computer on my truck tells me 59 mph is the sweet spot while towing on flat ground.

[dznf0g]

Originally Posted by crisen

Not to be too picky on this, well maybe, there isn't any change in the "nature of aerodynamic drag" the power to overcome aerodynamic drag (not aero drag itself) is a cube relationship to the increase in speed. What happens is that the power to overcome mechanical losses is basicly linear to speed. So at lower speeds these mechanical losses are the major part of the total power requirement to push any vehicle down the road. You can see that as the speed increases, with mechanical losses increasing only linearly and aero increasing as the cube of the speed increase there is indeed a point that aero is much more significant and this is where you see the increase in fuel consumption.

Ref: Bosch Automotive Handbook, 3rd Edition pages 324-326.

This is true and there is an additional factor over mechanical friction being linear. That is the design of the engine and the road speed/rpm where it is most efficient at combustion.

Been a long time since I have done the math, and it varies by rear end ratio and engine to engine, but you want the efficient road speed to be right where the torque curve flattens out in the gear which gives the best trans performance.

For the GM gas 4 speeds with 3.73 that I have done the math for it is at 63mph. Keep in mind this is the point where mileage really starts dropping of DUE TO COMBUSTION inefficiency. Wind drag is of course a bigger factor over say 55 mph. But you get the "double whammy" above the "sweet spot" for the engine.

[ROBERT CROSS]

POI....

Cruise control and MPG.

I've found on our 06 Burb that the "fly-by-wire" system makes using/not using cruise control redundant. MPG virtually the same either way.
Plus cruise set at 58mph on the Interstates, you will be hard pressed to pass anyone and it makes the long drives much less stressful.
I may hit the go pedal slightly on downhill sections to gain the up advantage but thats about it.

Bob

[SteveH]

Originally Posted by dznf0g

This is true and there is an additional factor over mechanical friction being linear. That is the design of the engine and the road speed/rpm where it is most efficient at combustion.

Been a long time since I have done the math, and it varies by rear end ratio and engine to engine, but you want the efficient road speed to be right where the torque curve flattens out in the gear which gives the best trans performance.

For the GM gas 4 speeds with 3.73 that I have done the math for it is at 63mph. Keep in mind this is the point where mileage really starts dropping of DUE TO COMBUSTION inefficiency. Wind drag is of course a bigger factor over say 55 mph. But you get the "double whammy" above the "sweet spot" for the engine.

I've tried driving 55 with my Duramax, and it doesn't seem to help any at all. I believe this to be because the Allison transmission coupled to the 3.73:1 rear end ratio will not go into sixth gear until about 62 MPH while in tow/haul mode.

Some of my friends say they don't drive in tow/haul, and get a little better mileage, but it's my thought that the whole truck performs better and it is easier on the transmission in tow/haul while towing the 31 footer. Otherwise, why would the engineers do it this way?

It would be nice to have a factory person chime in on this issue.

[Inland RV Center, In]

Originally Posted by Crawdad

I really like the idea of the air speed indicator also. Where can I find one and where is it installed?

Aircraft Spruce for the gauges.

Simply run some tubing from the gauge to the front of the vehicle where it protrudes from perhaps the grill by an inch or so.

The pitot tube must not have the wind hitting it altered in any way, such as from the design of the grill.

Andy

[flyfisher]

Several years ago, I slowed down from 65-67 mph to 60-62 mph when towing and found that over the course of several thousand miles my mpg had improved by 10%.

I then took that 10% savings, and divided it by the extra time I spent driving at the lower speed. I saved about $20.00 per hour. Fuel cost have increased significantly since I made that calculation, so the savings per hour would be higher today.

I figured that since I was retired, and in no particular hurry, anytime I could "earn" $20 an hour it was time well spent!

John

[dkottum]

When towing the Airstream, another thing that uses more fuel is the auto transmission shifting down on small, short grades on the roadway.

With our new Dodge Hemi I found keeping a speed that allows RPM above 2100 puts the engine in its higher torque range and it does not shift up and down. At 55-60 mph that means running in a lower gear, which is fine with me. Because it still uses less fuel at a constant rpm than shifting up and down for grades.

This is still dependent on wind conditions and direction.

And of course less wear on the transmission and engine.

doug k

[Phoenix]

Besides reduced drag, I think driving a little slower than traffic reduces the number of vehicles that you need to pass. Keeping up with the pack leads to frequent acceleration and deceleration from cruise control settings, which consumes extra fuel.

[dznf0g]

Originally Posted by SteveH

I've tried driving 55 with my Duramax, and it doesn't seem to help any at all. I believe this to be because the Allison transmission coupled to the 3.73:1 rear end ratio will not go into sixth gear until about 62 MPH while in tow/haul mode.

Some of my friends say they don't drive in tow/haul, and get a little better mileage, but it's my thought that the whole truck performs better and it is easier on the transmission in tow/haul while towing the 31 footer. Otherwise, why would the engineers do it this way?

It would be nice to have a factory person chime in on this issue.

Diesel and Allison (in particular) is a completely different animal than the gas.
The server is down for its weekly maintenance right now, so I can't pull owner manuals and service info. But, the general rule of thumb is when you are at 75% of the max load for your configuration, TH should be used. Below that figure, using TW doesn't hurt anything. May cause a decrease in mileage...but it is dependent upon terrain, wind, and what you perceive as pleasing.

Obviously this 75% figure is different in every setup. Example a 1/2 ton with max tow and a 30'er will need to be in max tow FOR SURE. a 2500hd Duramax with the same trailer...probably not.

[dznf0g]

Here ya go. This is for a Silverado





Tow/Haul Mode




Pressing this button at the end of the shift lever turns on and off the Tow/Haul Mode.


This indicator light on the instrument panel cluster comes on when the Tow/Haul Mode is on.
Tow/Haul is a feature that assists when pulling a heavy trailer or a large or heavy load. See Tow/Haul Mode for more information.
Tow/Haul is designed to be most effective when the vehicle and trailer combined weight is at least 75 percent of the vehicle's Gross Combined Weight Rating (GCWR). See “Weight of the Trailer” under Trailer Towing . Tow/Haul is most useful under the following driving conditions:

• When pulling a heavy trailer or a large or heavy load through rolling terrain.

• When pulling a heavy trailer or a large or heavy load in stop-and-go traffic.

• When pulling a heavy trailer or a large or heavy load in busy parking lots where improved low speed control of the vehicle is desired.

Operating the vehicle in Tow/Haul when lightly loaded or with no trailer at all will not cause damage. However, there is no benefit to the selection of Tow/Haul when the vehicle is unloaded. Such a selection when unloaded may result in unpleasant engine and transmission driving characteristics and reduced fuel economy. Tow/Haul is recommended only when pulling a heavy trailer or a large or heavy load.

[slowmover]

Originally Posted by Phoenix

Besides reduced drag, I think driving a little slower than traffic reduces the number of vehicles that you need to pass. Keeping up with the pack leads to frequent acceleration and deceleration from cruise control settings, which consumes extra fuel.

Yes, when FE is analyzed it is vital to not only limit the number and type of acceleration and braking events, but to reduce the steering wheel inputs. It takes energy to change lanes, even when done smoothly. Over the course of 100-miles this can really add up (according to both KENWORTH & CUMMINS).

The ideal is a steady-state speed which needs next to no throttle, brake or steering input. Which may not be as desired (that old teenager programming) as what is required to get from Point A to Point B is different. The balance between time & distance can be altered by the smart driver traveling more slowly, but with adequate safety stops of a predetermined length. Planning those long days by making legs of time/distance is workable.

Alternatively,

Let's say that our TT is one we will tow for 100k in the time we own it. A 20% reduction in fuel burn over that time might be 1,200-gls. At $4/gl that is close to $5k. And if our solo miles are done in like manner the savings may be double or triple that amount.

I found I could improve my solo mpg signficantly by combining errand trips. Fewer cold starts. Few, if any, short trips. After all, most errand trips take us the same places. Second, by doing a better job of driving those remaining miles the savings in fuel underwrites more than 5k of annual vacation miles. With the same fuel budget, "free fuel" for travel.

Fixed income? Ways to maximize that. Fixed length of time available? Ways to maximize enjoyment there. Fuel is just one cost, but it closely tracks wear & tear on my combination rig drivetrain & running gear and thus stands in for more than itself.

More than one way to skin the cat.

.

[dznf0g]

And me taking that 1986 Yamaha 125cc scooter along, which gets 94.5 mpg doesn't hurt either for those errands, beach runs, sunset runs.

[switz]

A rule of thumb from the Gas Embargo days of 1973 was a 10% reduction in fuel economy for each 5 mph increment over 55 mph. Thus at 65 mph, one could expect at least a 20% increase in the fuel burn. This statement reflects a still air environment. As Andy stated, headwinds will decrease MPG and tailwinds will increase mpg if the indicated speed is held constant.

The reason fuel savings did not materialize initially for the big trucks was the higher gears were setup for 70+ mph on the interstates. When the newer trucks came along with gears optimized for 55, then the fuel economy numbers improved.

My initial tow from the dealership in Los Angles to Phoenix at 55 mph generated a 17 mpg diesel fuel use and the engine was running at 1650 rpm on the level, right at the leading edge of the high torque band.

[Bill M.]

"My initial tow from the dealership in Los Angles to Phoenix at 55 mph generated a 17 mpg diesel fuel use and the engine was running at 1650 rpm on the level, right at the leading edge of the high torque band. "

What model truck?

thanks

[switz]

My TV is a 2007 Mercedes ML 320 CDI diesel. The engine is 3 liters with a 7 speed automatic and air suspension. It gets 27.5 hwy and 22.5 city.

[slowmover]

17 is my goal for the next trailer. We'll see . . . .

[Lumatic]

Originally Posted by Inland RV Center, In

A great addition to a tow vehicle, is an "Air speed indicator".


Andy

I agree the biggest fuel economy factor seems to be wind resistance. Followed by terrain features such as uphill tows.

An alternative is a manifold vacuum gauge. Tells you how much fuel you are sucking. A wind speed gauge (never used one) would not give you much useful information in situations like towing uphill. There are also mpg gauges but they cost more.

Just wondering. Has anybody played around with using an air deflector on the tow vehicle. The kind that mounts on the TV roof? How about building some kind of deflector similar to those now used on most tractor trailer combinations? Since Airstreams are somewhat aerodynamic anyway would it make a big enough difference to justify this addition?

[ROBERT CROSS]

"Just wondering. Has anybody played around with using an air deflector on the tow vehicle. The kind that mounts on the TV roof? How about building some kind of deflector similar to those now used on most tractor trailer combinations? Since Airstreams are somewhat aerodynamic anyway would it make a big enough difference to justify this addition?"

Yep.....

but it didn't add anything, then again....it didn't subtract either.

Bob