Depends on the terrain. But it's between 13-1350 RPM.

 

For those that may care, this discussion backs up what Music Man has offered as an explanation.
Bottom line is that it requires a lot more power (HP) to maintain a higher speed due to aerodynamic drag. And yes, weight is also a factor as is the frontal aspect of the vehicle and the density of the fluid (air).



Drag (physics)
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It has been suggested that windage be merged into this article or section. (Discuss)

An object moving through a gas or liquid experiences a force in direction opposite to its motion. Terminal velocity is achieved when the drag force is equal in magnitude but opposite in direction to the force propelling the object. Shown is a sphere in Stokes flow, at very low Reynolds number. F is force, d is drag, and g is gravity. Small arrows show direction of movement of fluid relative to sphere. Large arrows show direction and magnitude of equal and opposite forces on the sphere, which has stopped accelerating and is moving at terminal velocity.In fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a solid object through a fluid (a liquid or gas). The most familiar form of drag is made up of friction forces, which act parallel to the object's surface, plus pressure forces, which act in a direction perpendicular to the object's surface. For a solid object moving through a fluid, the drag is the component of the net aerodynamic or hydrodynamic force acting in the direction of the movement. The component perpendicular to this direction is considered lift. Therefore drag acts to oppose the motion of the object, and in a powered vehicle it is overcome by thrust.

In astrodynamics, depending on the situation, atmospheric drag can be regarded as an inefficiency requiring expense of additional energy during launch of the space object or as a bonus simplifying return from orbit.

Types of drag are generally divided into the following categories:

parasitic drag, consisting of
form drag,
skin friction,
interference drag,
lift-induced drag, and
wave drag (aerodynamics) or wave resistance (ship hydrodynamics).
The phrase parasitic drag is mainly used in aerodynamics, since for lifting wings drag is generally light compared to lift. However, the flow around bluff bodies is usually dominating enough that it is not considered parasitic drag since it forms drag, skin friction, and interference drag. Further, lift-induced drag is only relevant when wings or a lifting body are present, and is therefore usually discussed either in the aviation perspective of drag, or in the design of either semi-planing or planing hulls. Wave drag occurs when a solid object is moving through a fluid at or near the speed of sound in that fluid — or in case there is a freely-moving fluid surface with surface waves radiating from the object, e.g. from a ship.

For high velocities — or more precisely, at high Reynolds numbers — the overall drag of an object is characterized by a dimensionless number called the drag coefficient, and is calculated using the drag equation. Assuming a more-or-less constant drag coefficient, drag will vary as the square of velocity. Thus, the resultant power needed to overcome this drag will vary as the cube of velocity. The standard equation for drag is one half the coefficient of drag multiplied by the fluid mass density, the cross sectional area of the specified item, and the square of the velocity.
Wind resistance is a layman's term used to describe drag. Its use is often vague, and is usually used in a relative sense (e.g., a badminton shuttle**** has more wind resistance than a squash ball).

Contents [hide]
1 Drag at high velocity
1.1 Power
1.2 Velocity of falling object
2 Very low Reynolds numbers — Stokes' drag
3 Drag in aerodynamics
3.1 Parasitic drag
3.2 Lift induced drag
3.3 Wave drag in transonic and supersonic flow
4 See also
5 References
5.1 Inline
5.2 General
6 External links



[edit] Drag at high velocity
Main article: Drag equation
The drag equation calculates the force experienced by an object moving through a fluid at relatively large velocity (i.e. high Reynolds number, Re > ~1000), also called quadratic drag. The equation is attributed to Lord Rayleigh, who originally used L2 in place of A (L being some length). The force on a moving object due to a fluid is:

see derivation
where

is the force of drag,
is the density of the fluid (Note that for the Earth's atmosphere, the density can be found using the barometric formula. It is 1.293 kg/m3 at 0 °C and 1 atmosphere.),
is the speed of the object relative to the fluid,
is the reference area,
is the drag coefficient (a dimensionless parameter, e.g. 0.25 to 0.45 for a car), and
is the unit vector indicating the direction of the velocity (the negative sign indicating the drag is opposite to that of velocity).
The reference area A is often defined as the area of the orthographic projection of the object — on a plane perpendicular to the direction of motion — e.g. for objects with a simple shape, such as a sphere, this is the cross sectional area. Sometimes different reference areas are given for the same object in which case a drag coefficient corresponding to each of these different areas must be given.

In case of a wing, comparison of the drag to the lift force is easiest when the reference areas are the same, since then the ratio of drag to lift force is just the ratio of drag to lift coefficient.[1] Therefore, the reference for a wing often is the planform (or wing) area rather than the frontal area.[2]

For an object with a smooth surface, and non-fixed separation points — like a sphere or circular cylinder — the drag coefficient may vary with Reynolds number Re, even up to very high values (Re of the order 107). [3] [4] For an object with well-defined fixed separation points, like a circular disk with its plane normal to the flow direction, the drag coefficient is constant for Re > 3,500.[4] Further the drag coefficient Cd is, in general, a function of the orientation of the flow with respect to the object (apart from symmetrical objects like a sphere).


[edit] Power
The power required to overcome the aerodynamic drag is given by:


Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

It should be emphasized here that the drag equation is an approximation, and does not necessarily give a close approximation in every instance. Thus one should be careful when making assumptions using these equations.

 

man.. talk about learning a foreign language..

 

Yes heavyhaulerss my 3406 was on the dyno at 636HP way back in the last century. I went back to stock nozzles - much better fuel mileage.


550W900L I like the smoke. Mine looked like that with the hot nozzles.


All this theory about fuel mileage and nobody mentioned keeping an eye on the boost gauge!

When I hear the turbo spool up I have visions of $$$ flying out the stacks.

 

I don't even have a boost gauge in my truck Tried to get our shop to install it, but they said it'd be too expensive unless I "get a kit". How do you determine which boost number on the gauge is optimal and which one is too much? Is it different for various engine makes?

 

The less you got, the less fuel you are using. And yes, it's different for various engines. On my 3406E, the highest, i ever got, was about 32 psi. My current C-15 Acert, I've run up to 50psi, and not even sure, it's the limit.

 

tracer a junkyard will sell a gauge for $5. A new one should be around $30. It doesn't even need to say "intake press" on it. As long as it goes from 0 to 50 psi.

Some 1/8" plastic tubing and brass fittings - $15 maybe less.

 

I wasn’t commenting on any one person's specific situation. I was using recent average fuel prices, not what it is right now. Were you paying $2.45 a gallon in July? Do you honestly believe fuel prices are not going to go back up? If you base your financial decisions on $2.45 a gallon fuel, you are foolish. I was also using a decent average mpg for a non-aero truck (the type that would have external cans) and a decent average of number of miles run for a solo driver who has the opportunity and likes to drive. For God’s sake, I could have used my own numbers… 6.8 mpg, average $2.23 a gallon for December, and 230k miles a year if I wanted to be specific. But I have an aero truck and run more miles than the average truck does, so that wouldn't have worked for this discussion.

First I would like to point out that you directly contradicted yourself. First you say that according to Kenworth, external cans cost 1% to 2% in fuel economy, and then you try to argue that they do not affect economy because of all that frigid “quality” air they suck up. Which is it? Is KW correct when they say they cost fuel economy, or are we to believe YOU when you first cite KW in attempt to support your argument and then go on to proclaim that KW is wrong and you are correct? I’m confused. External air cleaners do provide more air although it is arguable how much more, which is a great thing… if your engine is starved for air. If it is not starved for air, it will make little or no difference in economy or power. Same thing applies for less restrictive exhaust, by the way. Pittsburg Power is a great company, but they are not Gods, and they do want to sell their products, so I wouldn’t blindly accept everything they say. Let’s do a little independent thinking here: where do firewall mounted air filters get their air from? Seems to me, the last time I looked under my hood – which I have to admit was yesterday, since I am home for the holidays – I noticed that the air cleaner canister has an open hole at the top of it. The hood has a built-in air duct that fits securely over the opening in the canister at one end and opens up directly into the clean, cold, refreshing air that rushes past the side of the hood. Now, I have to admit that I do not know exactly how much air my engine takes in each minute (I’m sure someone here can answer that for a 450HP ’06 ISX) at cruising speed, but I’m sure we can both agree that it’s a hell of a lot. How much do you really think that air heats up in the fraction of a second that it spends inside that under-hood air cleaner before it is moved into the intake? I’m sure you really don’t know and neither do I, but a tiny amount of common sense and a teensy amount of science knowledge would allow us to deduce that it couldn’t be more than a degree, if that much.

I’m not sure why you would say this, other than that is how it was for a long, long time, until fuel skyrocketed. My wholesaler friend (John, if you are reading this, feel free to chime in) confirms that while fuel was over $4 a gallon, you couldn’t give a long-hood away. Nobody wanted them. He talked to many dealers who were trying to dump these trucks for next to nothing, and were jacking up the prices on aero trucks at the same time. Now that fuel has gone back down, many geniuses are starting to covet the fuel hogs again. But that’s right, fuel is $2.45 a gallon and will never again go up, right? Some people never learn, and I won’t feel sorry for them when they can’t afford the fuel for their 5 mpg trucks when it does go back up, and it most definitely will.


If you say so. Grossing 80k I can average 6.7 mpg running at 62 mph… as long as it’s not between Portland and Redding, or heading west on I-80 (usually into a 30 mph headwind) from Lincoln to SLC. Then I might average 5.8 to 6.2 mpg. Grossing 60k, I will average 7.4 mpg, except in the aforementioned bad areas, in which my mpg will drop to between 6.2 and 6.8 mpg. In my experience, speed (which primarily means WIND resistance), and headwinds affect mpg more than anything.

 

Musicman, great posts . You seem to have explained this much clearer than I have...

 

Thanks, Tom.

Before everybody starts jumping on me for being a long-hood hater, let me say that I am not. I love the looks of many of the “classic” truck bodies. I do believe, however that they are no longer a good choice for most of us who are not running specialized freight such as over dimensional, open-rack auto transports, or using our equipment primarily in off-road applications.

If you are truly running a business, you should make decisions that maximize profit by reducing expenses and increasing revenue. Since fuel price per gallon has gone up and become much more volatile, the easiest way to reduce expenses it to control fuel consumption. In applications where the majority of time and miles are spent driving long distances at a steady speed, the only way to go is the most aerodynamic truck design you can find.