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Old 10-28-2005, 07:42 AM   #57
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The laws of physics, and the definition of horsepower, say the torque and horsepower intersect at 5,252 rpm.

One horsepower is 33,000 ft-lb/minute. To convert linear feet to rotation, you divide by the circumference of a one foot circle, 2 times pi feet, which results in 5,252.

HP = (torque x rpm)/5252

Torque = (HP x 5252)/rpm

HP = Torque at 5,252 rpm

You can make the lines on the graph, but not the values they represent, intersect at ANY rpm you want, simply by changing the vertical scales of horsepower or torque. Unless the scales are the same on each side, where they intersect is meaningless.

Best efficiency typically occurs at the torque peak, where the increasing volumetric efficiency from air in motion intersects with the decreasing time of intake valve opening. The problem with gas engines, and some modern diesels, is that the torque peak occurs way above the speed you normally drive at.

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Old 10-28-2005, 08:29 AM   #58
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Something I haven't seen discussed is headwinds and tailwinds. I keep a good eye on weather and winds when I travel. If I'm expecting strong headwinds on travel day, I will try to get on the road very early when winds will be the lightest. Typically, morning winds are light and wind speed tends to build all day until just before dusk. If I am expecting very strong headwinds, I will opt to postpone travel for a day or two to let the fronts pass.

If I'm expecting tailwinds, I will delay departure a couple hours to give the winds time to build. On our last vacation, I was able to achieve our best gas mileage, 16.5 mpg, going from Big Lake, Texas to Amarillo, a distance of 365 miles. We had a 20-25 mph tailwind and we traveled at 55-60 mph. It was the equivalent of pulling the trailer at 40 mph. It required almost no effort at all.

My experience has been that "wind resistance" not "weight" has the biggest impact on gas mileage. I get slightly better gas mileage pulling our '97 Excella 25 than I did pulling our previous trailer, a 1994 Award 23 which weighed about 3200 pounds. The Award is considered as aerodynamic.

A note about tire pressure. For optimum mileage, I run my tires at or just below the highest recommended air pressure. It may give a bit rougher ride on rough roads, but the rolling resistance is lower. Most tires fail because of running too low of pressure, so higher pressures benefit the tires also.

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Old 10-28-2005, 08:39 AM   #59
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One of my customers installed a propane conveersion on his Ford 7.3 PSD, among several other upgrades.
He now has a 700hp engine, with 1000 lb/ft of torque, and gets 21 mpg while carrying his truck-bed camper. And it only cost him $20,000, including the new transmission we had to upgrade/install to handle the higher figures.
The transmission was $3500 installed, the propane injection system was $2200 installed, he got a Banks turbo and exhaust, and other goodies I have forgotten.
The down side is, the truck sounds like an F-16 driving down the street.
Boy, am I glad I have the technical and mechanical expertise and experience to do these things myself. My complete propane injection system cost me only $550 including the tank. As far as transmission upgrades are concerned, if one drives "normally" they are not needed. A lot of guys do these upgrades then hammer the hell out of the vehicle. You have to pay attention to what is going on and not drive blindly, which most people do. (over 30,000 motor vehicle deaths a year prove that ) They steer in the direction they want to go, put the hammer down, turn the radio up, ignore the sounds of the engine and ignore what the gauges say. To me paying attention to all this stuff is half the fun of driving, to others it is is not.

In a nutshell, propane injection can net you an additional 1-3 mpg, plus 85-100 hp on a Duramax. This all hinges on the cost of the propane, of course. If you can get propane for about 66% of the cost of diesel fuel, you can save a few cents a mile, and increase your cruising range, if you have a large enough c3h8 tank.
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Old 10-28-2005, 09:06 AM   #60
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[quote=ipso_facto] The Exhaust Gas Recirculation valve is an emissions device designed to improve mileage, and it goes bad from age and gets carboned up.

A little correction here. The EGR valve was not designed to improve fuel mileage but rather was a emission control device thats purpose was to reduce NOX., nitrogen oxide emissions. It was one of the first systems that were piled on the big inefficient engines of the 70's. It along with air injection pumps, heated grids under carbs., air filters with manifold heat tubes, ignition timing retarding devices,etc were just some of the so called quick fixes the industry used to try reduce emissions the feds demanded of the auto industry. Some worked, some didn't, most killed horse power and increased fuel consumption. I worked in the sevice industry through all those years and can tell you from first hand experience that it was a nightmare. Complaints of hard starting, stumbling, stalling, dieing, no power' poor mileage we just some of the complaints new car customers had of those days. On top of that there was very little we could do to help. Emissions testing and threats of huge fines kept all but a few of the brave from removeing them.It wasn't until the computer age that we begun having the best of both worlds, that being clean emission, good fuel economy, and good power from much smaller engines. When people say "they don't makem like they used to" my reply is "Thank Goodness!!! Back to the EGR system. This is the only one of those original devices I can think of that is still being used. The purpose is still the same as originally intended ,to reduce emissions. However it does have a secondary result in that it does improve fuel economy. End of discourse.
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Old 10-28-2005, 11:07 AM   #61
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Originally Posted by ipso_facto
I hear that best MPG is where the HP and torque curves intersect. For my 7.5L engine and auto, it's 2200 rpms. Which usually means 47mph. I have gotten almost 12 mpg once like that. Usually it's 10 empty.

I would love to own a 5 speed due to reduced rpms. 3 speed automatic and big block does not lead to good mpg.
A Gear Vendors under/overdrive would work good in your case. I was looking at their products on their website and it sounds like a pretty good system, especially if you've got a 3 spd.

I have the same engine but with a 4sp auto with 4.10 gears and I get about the same mileage 10.5 to almost 12 empty and about 8.5 towing. At 60 in OD I'm running about 1850 rpms.
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Old 10-28-2005, 11:47 AM   #62
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fuel additives

i realize this is a big can of worms and i'm as doubting as most educated folks when it comes to a magic juice.....but

i've used the stanadyne all season performance formula fuel additive with positive mileage results.

my powerstroke is still new (6 months/16,000 miles) and i decided to try the stanadyne at about 500 miles into ownership. with long trips, tires maxed and no trailer in tow it seemed to boost mileage 2-2.5 mpg or so on the highway (i alternated tank fulls for 3000 miles) and less with mixed/city driving...1 mpg or so.

with towing these last 11000 miles it still seems to improve mileage....although less so when towing at higher speeds.

when diesel was 2.20/gal the stanadyne wasn't really a value but now with prices at >than 3$ it is....there is a local shop that sells the 300 gallon/treatment jugs for 24$.

i understand the antijelling/anti fungicide issues with diesel fuel additives and the lubricity issues with very low sulfur fuels and the claims for cleaning injectors, but ....

anyone else increased mileage with an additive?

i duck now under the trailer......


i've occasionally tried a tank with some other additives and not recorded any mileage benefit.
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Old 10-28-2005, 03:22 PM   #63
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A Gear Vendors under/overdrive would work good in your case. I was looking at their products on their website and it sounds like a pretty good system, especially if you've got a 3 spd.

It would. I have the 3-speed tranny. I've been relucant to get it, due to the fact that it's expensive, and given that I drive so rarely, it would take a decade to pay for itself. Secondly, it is a proprietory part, what happens if it breaks down - no easy replacement. Thirdly, it doesn't solve the real problem - automatics are less relaible than manuals for towing. Less heat generated. For me, the real solution is a 5-speed. I realized that after owning autos all my life.

Really, there must be a way to decrease the cruising RPMs, at 60, I am turning almost 3,000 which wastes gas.
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Old 10-28-2005, 04:20 PM   #64
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Re: Gear Vendors. I have an uncle who owns a fleet of vehicles, in addition to a used truck dealership. When he buys a new truck for himself (always a 1 ton dually with crew cab) the first stop he makes is at Gear Vendors. I have never been willing to spend that kind of money for one, though.

Re: Additives. With my old Chevy 6.5 turbo diesel the Stanadyne additive gave me roughly a 5% improvement in mileage - from 19 mpg to 20 at highway speeds. I've tried it in my old Mercedes 190, but can see no improvement. I've yet to try an additive on the new Duramax. Somebody will have to sell me on the whys and wherefores of an additive with that motor.

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Old 06-19-2008, 08:16 AM   #65
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Changes to Tow Vehicle to improve Mileage

This has been touched on in other surveys, but I thought it would be good idea to put all the ideas in one place.
What are you doing, besides the usual things, to improve gas mileage on your TT and have they improved your mpg ?
I am looking at several different aftermarket items: electric radiator fan, new exhaust system and new air intake.
Rick and Joanna
On the road, cause she's always telling me where to go.
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Old 06-19-2008, 08:39 AM   #66
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Keep clean oil in it, keep the filters clean, keep your tires at recommended pressures and drive the speed limit. Other than that, don't waste your time searching for the "X" that marks the spot of the fabled fuel economy treasure chest. It doesn't exist or every automobile manufacturer in the world would already own a share it.
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Old 06-19-2008, 09:05 AM   #67
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I drive a diesel pick up tow vehicle. I have had a couple over the years and noticed that they tend to have a "sweet spot" mph where they seem to run at the best economy and power. Find that and it might help you maintain good fuel economy. For my present truck it is about 60 to 62 mph. The only major mod I have done to my trucks has been to add larger dia. exhaust and flow thru muffler. But, that was mainly to lower EGT's when towing up hills. I didn't see any milage improvement from doing it.
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Old 06-19-2008, 09:11 AM   #68
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What towing speed is best?

How fast, or slow, as the case may be, somewhat depends on the wind.

How efficient an aircraft flies, is very important on the wind speed and direction.

Installing an "Airspeed indicator" in your tow vehicle, will tell you if you have, so to speak, a tail wind or a head wind.

Obviously, if you have a head wind, it will impede your towing mileage.

When you have an airspeed indicator in your tow vehicle, you simply compare the readings of your speedometer with it.

If your speedometer indicates a speed higher than the airspeed indicator, then you have a "tail wind", which tells you that you can tow faster.

If your speedometer indicates a lower speed than the airspeed indicator, then you have a "head wind", which tells you to drop your towing speed.

What speed to tow at, then becomes a subjective opinion.

Trial and error will quickly tell you what is best for your particular rig.

I personally, increased my towing mileage by using an "airspeed indicator," as a guide, sometimes by as much as 20 percent.

Not bad when you consider todays fuel prices.

We all ignore wind speeds moving on the ground, but every aircraft pilot is acutely aware of wind speed and direction. That's why they usually change altitudes, so as to maximize fuel efficiency, and to minimize time enroute.

Since it works very well for aircraft, it works very well for traveling the highways with a motor vehicle, towing or not.

Try it. You won't like it at first, since it tells you whats going on, and what to do, like it or not.

But, if you really pay attention to it, in short order, "you will love it."

Anything that you can do to increase highway fuel economy these days, really pays dividends.

I know, as I took advantage of that setup, starting in 1970.

Since it worked very well then, it certainly would work equally well, or better, today.

An airspeed indicator can easily be installed. Simply run a 1/4 metal tubing from it, thru the firewall, and forward of the grill at least an inch or two. Fasten the tubing so that it will not easily move when traveling. That acts, if you will, like an aircraft "pitot tube". A static line is not required, since the accuracy of the reading is not as important, as the difference in it's reading compared to the speedometer, which is all important.

Come on pilots, chime in.

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Old 06-19-2008, 09:23 AM   #69
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Wasn't there another thread about the airspeed install somewhere?
Yup I'm gonna put a ASI on the dash of the burb soon. Got a spare one on the bench.
You know Andy- once I started using a GPS while flying I was all over the sky looking for that favorable wind! It's truly amazing what 100 of altitude will do to improve the fuel burn or should I say speed made good or distance made good. Now I'm getting the nautical, aeronautical terms intertwined. Oh yea that manifold pressure thing too

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Old 06-19-2008, 09:40 AM   #70
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I am a pilot and, to be honest, you will have to explain to me how knowing the wind will improve gas mileage on the ground. In the air you have a three dimensional environment to operate within so you can adjust your altitude to find more favorable winds. On the ground you don't have that option so, if you intend to get from point A to point B by the shortest overland route you are pretty well at the mercy of the winds. If the shortest route puts you into an unfavorable wind I suppose you could always choose a different route but then are you really saving anything by increasing your mileage between those two points?

I don't get it.

Personally, I think your attention needs to be focused on the road and the traffic around you when you are driving, not on another gizmo or gadget in addition to the cell phone, GPS and God knows what else people are looking at other than the road.


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gas mileage, tow vehicles

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