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Old 12-15-2012, 10:41 AM   #15
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I was still in school when President Nixon signed the double nickel law in 1974. Interesting reading how the mileage savings weren't so good -- many states wouldn't enforce the 55 limit as tightly as they had at higher speeds. So yes, it certainly does come down to actual ground speed.

I'm still mulling over Andy's airspeed indicator. This could get positively Rube Goldbergian...
Bob.

I used the Airspeed indicator for 4 years, when I was with Caravanner Insurance.

In spite of guesses, opinions, and gut feelings, it's use made a huge difference in fuel mileage.

Little do we suspect but as we travel across a given area, the wind speeds do change as well as from different directions.

It's used also very clearly demonstrated what happens to the air currents when meeting an oncoming vehicle. That quickly explained why people sometimes lose control of their rig, because of that increase in wind speed, especially when unexpected.

Airspeed indicators are not really that expensive, nor difficult to install. Certainly, even with a moderate amount of travel per year, the use of one, more than pays for itself in short order.

It also quickly teaches the driver of how quickly something that we cannot see changes, which really suggests changing our driving habits.

Most folks are not aware that if you meet an oncoming vehicle that is going, as an example 60 miles per hour, that speed momentarily adds to yours, and then oscillates downward. If not rigged and adjusted properly hitch wise etc, you could be in very sudden trouble.

I call it's use, "a guide to efficient and safer towing".

Andy
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Old 12-15-2012, 10:52 AM   #16
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Better believe it! Twenty-five years ago my father in law had an experience pulling a 19' SOB with a '70s Buick sled into the teeth of a gale in SE Colorado. He got 4 mpg amidst some very tough (and not very fast) driving that day. They found that renting snowbird accommodations in AZ & TX was just about the same $$ as camping and sold the trailer after that trip.

He'd always speak about how we could pick up an inexpensive trailer in those locations from people who didn't want to drive it back home. Airstreams? Nope, no Airstreams among them... Guess I just had the bug.

BTW - lack of hills and not using cruise control are a great contributor to better mileage.
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Old 12-15-2012, 11:44 AM   #17
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But it does seem to help fuel milage in the mountains to cost up in speed going downhill and let the speed drop going uphill. On my diesel if I set the cruise at 55 it is a real pain in the hills because it slows down just before it starts up again.
I went to the west coast and back in a Volvo pulling a pop up back in the double nickle days. I do not know what it did to the mpg, but it sure took a long time. 60-63 on level ground is actually a good number with my current rig. My biggest deterent to mpg is stopping frequently. Each rest area seems to drop the mpg meter 3 mpg or so and it takes a long stretch for it to build back up. Also uses a lot of fuel in the campground parking.
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Old 12-15-2012, 11:47 AM   #18
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Starts and stops are a real killer for me too! Once I get going, I try not to stop until I need fuel, or it gets close to quitting time.
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Old 12-15-2012, 06:40 PM   #19
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From an undergraduate physics class, there's a change in the nature of aerodynamic drag on a vehicle that occurs around 60 mph. Up to the change, the drag has a linear relationship to speed, above the break point the drag changes to a square relation. Below the breakpoint the drag increases in proportion to speed and above it the drag increases in proportion to the speed squared (goes way up with a little increase in speed). That's exactly why Pres Carter lowered the national speed limit to 55 mph back in the 70's during the oil shortage, because slowing to that speed uses a significantly lower amount of fuel.

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.
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Old 12-15-2012, 08:17 PM   #20
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I just experience this today. It was the first day of our 3200 mile trip to Florida. We drove all day in a horrible rainstorm (Northern Ca.). While crusing down I5 I glanced at the computer and saw 9.2MPG. I looked at the speed and it was 62. The flow on I5 is 72, so I was hesitant to get too many guys up my tailpipe. Anyway, I slowed to 55 and the computer jumped to 11.3MPG. Of course, this was a crude measure just using the current MPG gauge, but I think the concept was pretty clear.

I think we were also into a headwind with the rain too. I sure like the idea of a airspeed gauge.
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Old 12-15-2012, 08:42 PM   #21
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I really like the idea of the air speed indicator also. Where can I find one and where is it installed?
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Old 12-15-2012, 11:12 PM   #22
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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.

.
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Old 12-16-2012, 07:24 AM   #23
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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.
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Old 12-16-2012, 07:46 AM   #24
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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.
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Old 12-16-2012, 07:48 AM   #25
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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
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Old 12-16-2012, 08:46 AM   #26
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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.
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Old 12-16-2012, 09:26 AM   #27
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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
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Old 12-16-2012, 09:54 AM   #28
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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
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