Sorry guys but you will never convince me that speed is a factor in gas mileage. I used to work on a Nascar Busch team and we would get about the same gas mileage at 170mph as 120mph. The reason being the engines, trannies and rear's were built completely different for each of those speeds.

You can do 100mph with 60hp just like you can with 800hp. The difference is how long it takes to get up to that speed. With a short enough final gear you would only need a small amount of HP to keep a car rolling at 100mph while crusing at 1000rpm.

So you guys can keep arguing about it but speed only matters when you talk about one individual vehicle. A slower speed may get better mileage for one application but may actually increase it in another, and that's the bottom line.

I don't really give a rats ass who you worked for. Test it for yourself, whatever way you can. I'm lucky I have a computer that spits out instant MPG. I notice a huge difference (1-1.5 mpg) between 60 mph and 75 mph.

You're pushing more air, more air requires more horsepower, and horsepower requires more fuel. It doesn't get any more basic then that.

If fuel mileage was better at higher speeds, then logic states it requires less horsepower to run at higher speeds. So you're telling me when you "back off", your vehicle speeds up? Please, get yourself out of the hole you've dug while you can!

It doesn't matter if you have 60 hp or 800 hp at 100 mph, what matters is what the engine is making at that time. If it takes all the juice of the 60 hp engine, then obviously at that time it's making 60 hp. The 800 hp will also be making 60 hp cruising at 100 mph.

You know... pressing on the throttle more creates more horsepower, less throttle = less horsepower.

also, slow that 170 mph car (I assume it's a restrictor plate) down to 120 mph, and suddenly it will get better fuel mileage.

The 120 mph car is losing a lot of mpg because first of all it has more horsepower, second it needs to bleed of speed going into the corner. This wastes momentum that you need to use fuel to make up.

You're comparing apples to oranges.

Take any one car, one car, and it will get better fuel mileage going slower.

I agree overall, but I have to wonder how gears and rears fall into this. They have to matter somehow. I can run 60 in top gear on flat ground, but if I hit any kind of hill, I have to drop one, and raise my RPMS by 500. I'm moving the same air either way, so it's more efficient to stay in top gear, but if I hover right at this breaking point, I wind up changing gears constantly, which will put more wear on my transmission than if I pick the gear it will run in at this speed (9th, direct) or increase my speed slightly to get away from this margin, and reduce shifting.

I also think there must be a coast factor. If I run 65, I can come up the hills in top (10th, overdrive) gear 70% of the time, then I let off at the top and with no fuel on the pedal, I think the engine computer considers this a zero fuel state, so I'm not burning anything on the descent. I kick in the Jake to hold my speed, and I don't think this burns any fuel either on a modern ECM-controlled engine, and then I get back into it hard in the bottom third, to try to come back up the next one at 65.

I think this must save fuel over running 60 in direct, and save tranny wear compared with running 60 in the highest possible gear in any given situation, but I don't have a computer in this truck, so I don't get realtime feedback.

I don't pay for the fuel anyway, and I don't really care that much. The O/O I drive for seems to be a lot more of a fuel guzzler than I am, with his big W900L that he lets idle all the time, so I'm not worried.

I'd need more proof than someone merely telling me on here that 30-35mph is the most efficient speed ina car. All of my trucks (suv's) never kicked into 4th until 40, turning a lower rpm.

"Well, I think its true, so it must be" mentality :wink:

Those on board computers aren't accurate, so go ahead and keep swearing by it. It's such common sense that I can't even dumb it down for you to explain that final drive ratio means more than speed and horsepower.

Buddy I'm the one "dumbing it down".

Anyone with half a brain knows the horsepower required to push air increases exponentially with speed.

changing diffs won't magically get you back a ton of fuel mileage unless the engine is wayyy out of it's range.

Also, you're comparing two completely different cars. And no, not just a "diff change" either.

And yes, the onboard computers are fairly accurate. They know how much fuel your injecting, and they know how fast you're going.

They might be off(mines about 2-3% off) but they're off by the same amount all the time. They don't magically change.

You're going completely against the grain of what is common knowledge in our business. There's a reason big companies run a bit slower then the average joe. Do you think they're doing this for the hell of it?

Until you offer some proof, you're just arguing for the sake of arguing. There's tons of white papers, even aerdynamic wind tunnel testing showing the gains of running slower.

Get over yourself allan, running slower does not always mean better mpg and that is all there is to it. You are not always right, and it is obvious you just like to argue. Have a nice day and be sure to stay out of the middle lane while you "saving fuel".

If I'm not right about this, counter my points, and refrain from ad hom attacks.

I have countered your weak points already there is nothing more that I need to counter. And if you think I'm attacking you, then you are definitely in the wrong profession.

You have offered nothing but your opinion and a reference to an article that you read somewhere and that is all. If going slower saves you fuel in your vehicles that is fine and dandy. But that is not always the case, just like in my 97' ford dually power stroke, i got better fuel mileage at 72 than I did from 60 up to 72. So hmmmm looks like you were right.

As I said get over yourself, you do not know everything. I know it's hard to believe being a trucker and everything but the sooner you realize that the better it will be for everyone.

Well, Jegzus, if you are right, than the physics laws are wrong! :wink:
Yes, the proper specs is very important, but the faster you go, than more you spend!
And please, don't tell me to stay out of the"middle" lane.
I'll stay wherever i feel comfortable at the moment! :P

Your wrong Solo. I just got off the phone with NASA engineers. Let me repeat that, NASA ENGINEERS. They told me the Space Shuttle burns the same amount of fuel at 3,000 mph as it does at 50,000 in outer space.

Lets see you explain that one smartie pants.

I race a car at the local dirt tracks. I race at 1/4 mile tracks and half mile tracks. On the quarter mile tracks you put a lot bigger gear in, I usually put a 6:00-6:20 in and turn 7500-8000 rpms. On a 1/2 mile I run a 5:14 or 5:29 and try to turn the same rpm's, but I don't reach them near as quickly. I watch my fuel closely. I can tell you that I get the same fuel mileage in both. It all depends on how hard you are on the throttle. RPM's are what burns fuel. I have ran a 5:43 at the quarter mile track, I did this as a test, I was trying to burn less fuel to keep more rear percentage at the end of the race. Believe it or not, with that gear I burned about 3 gallons less in a 20 lap feature. Why? Because the whole feature I turned 6700 rpms. Now that is like comparing apples to oranges however. I am hammering the throttle, letting off, hammering and letting off. In a truck you are accelerating and in most cases keeping it there. In a truck, the harder the engine runs, the more RPM's you turn. To go faster, you push down the accelerator harder so it is going to burn more fuel. I'm sure you weren't running the same gear in the 120 mph car and the 170 mph car. You were probably running a taller gear in the 120 mph car, going for quick power and running a shorter on in the 170 mph car for speed. Put the 120 mph car on the track that the 170 mph car was running on and run some laps. If you don't blow the motor due to RPM's, you will notice you burned much more fuel.

So Steve...just cause you have a Star car doesn't mean you get to call NASA direct...:)

Now....hold your hand out the window palm facing forward at 30mph and 70mph. Takes a lot more "energy" to hold that palm there doesn't it. Wind resistance increases geometrically with speed. Therre is simply NO WAY that it takes less energy to go 70 than 60.
However the gearing, torque, and rpm's certainly impacts fuel consumption. If you geared your truck to run 70 then you will probably consume less fuel running 70 in the sweet spot of 13th gear rather than screaming along in 12th at a higher rpm but lower speed.

That's easy! There is no air, at "outer space", therefore, no resistance! :D
Here, on Earth, even outside temperature, affects air resistance!

And one more thing, about the "same MPG" at the same RPM, no matter what speed;-
I'm going 1500 RPM 70 mph, up, or down the hill.
Does it mean my MPG, is the same? :P

There is some resistance from particle friction even in a synchronous or asynchronous orbit. The International Space Station has to get a boost from a visiting space shuttle to insure the whole thing won't slow down and come crashing down (like MIR).

Steve's point is that the velocity of an object in space is unrelated to the fuel consumption needed to accelerate the object. The acceleration is constant, therefore the 'miles per gallon' is constant. The difference between starting velocity and target velocity determines how much fuel needs to be burned to reach the speed desired, but the fuel flow is the same.

In the physics of a truck, one must remember Newton's 1st law of physics. An object in motion tends to stay in motion unless acted upon by an external force (this is called momentum).

80000 lbs in motion will stay in constant velocity until other forces such as mechanical friction, air friction, and the force of gravity act on the truck. Pretend you take a truck out of gear and let it coast (don't actually do this please), and see how long that truck will take to slow down. It'll take minutes before you come to a complete stop on a perfectly flat plane. Therefore, air resistance & mechanical friction: NEXT TO NIL.

YES IT DOES. Have any of you called NASA???? NO!!! You don't have a WS.
All this talk about physics, motion and particle MIR space stations is hurting my head. I'm gonna go out and floor my truck.

Sorry about that Steve.

Here have an ice pack, on me. :)

Imagine that there are other people out there that understand what I was talking about. It's more about the right set up than the speed. 8)

And solo I wont tell you to stay out of the middle lane but don't start crying on the radio about people passing you on the right. :roll:

I don't care! It's free country, you could burn tour money, or do other stupid things...it's all legal! :P :lol:

Good deal then! I do hate that though when you ask someone doing 60 in the middle lane to please move it over so the people going the speed limit or faster can get by with out passing on the right and they give that "I'll drive my truck the way I want" stuff then bitch at you when you pass them on the right.

You can't argue with a fool.....the faster you go, the more you burn...Don't argue with a pig.....he enjoys it.... :lol:

That's not me! If I'm in the "middle lane", i do the speed limit, or "traffic flow" speed. Whichever is lower! :D And if it's not congested, I'd move to the right.
But you wouldn't believe, how many times, i watched the same trucks(cars) had passed me over, and over again! :roll: :wink:

There is an element of truck specing that plays a role in this. With the way TMC trucks are set up I get much better fuel economy at 65 than 55. Running at 55 means running in a lower gear, so the engine is turning just as fast and using the same amount of fuel per hour as running at 65, but you have to take ten minutes longer to travel the same distance. That extra ten minutes burns up more fuel balancing out the advantage caused by less wind resistance. If you specced the truck to obtain max fuel economy at 55 then you would see an improvement.

The biggest difference that I see in fuel economy is the load weight. If I am loaded to around 70,000 pounds gross I get better than 6 mpg. Loaded at 79,500 or so costs a quarter to a half a mile per gallon.

Interesting article...older (2005) but I don't think the principles have changed

http://www.truckstop.com/ITmagazine/pics/22.jpg

I hear ya Solo, but you don't have to worry about me passing you a bunch of times. I'm a day cab driver so I don't stop until I get to where I'm going, and most of the time I run 70-72 which is the flow of traffic around here. But most of the over the road guys are the ones in the middle lane doing 58-64 and they get pretty nasty when you ask them nicely to move over.

I've been looking to buy my first truck for the last 3 years. Saving as I go.. and while I've been doing this I've made it a point to get the best fuel mileage I can. I'm still a co. driver lol

This is what I've found take it for what its worth

Spec's do play a roll. So doesn't speed. They go hand in hand. I've driven Long nose petes with 550 cats under the hood. 18 speed tranny and pulled between 4.8 and 6.6 mpg. This was a car hauler. Truck was geared for speed . 4.8 mpg was attained while spinning the turbo all the time. Empty, loaded didn't matter. I let off the throttle and the mpg would dramatically go up. Alot of people (my self included at the time) don't realize you don't need as much fuel as were feeding the engine to keep the truck in motion. Some people get hooked on the sound of that turbo, and just keep it spinning. I realized it wasn't gonna kill me if I crested a hill @ 60 vs 65 mph. If your heavy, climbing a hill.. not much you can do.. but attacking the hill correctly ie: correct gear, speed blah blah. Stuff we all know. But spec's and speed both play a roll. It's not one or the other.

A truck geared for speed will yield real nice mpg#'s when the avg weight is low. Truck needs to geared for area being run, weight it will haul etc etc. We all know this.

A truck geared for a mix of speed and pull will yeild similar #'s as the truck geared for max speed when pulling heavier load. As long as it's driven correctly.

Simple fact is.. Driving slower has proven to be a easy simple solution to gaining better mpg. Driving a properly spec'd truck will do even better when driven properly.

It's no secret that a higher horsepower engine doesn't work as hard as a lower horsepower engine pulling the same weight.

My company pulled my out my Columbia and gave me a T800. In the 4 months I drove the Columbia my avg wgt was approx 38k lbs. My 4 month MPG avg was 6.84 mpg. Less then 10% idle time. I reguarly drove 62-65 mph.

Columbia (areadynamic)sp?!
DET S60 475/515
Direct 10 speed
3:56 rears.. (will have to double check that one)

The KW I'm in now.. still avg about the same weight, And for the last 4 weeks I've been able to squeeze an avg of 6.6 mpg out of it. My idle % has avg 15% due to heat and the need for A/C. This truck is limited to 64 but still keep it within the 62-64 mpg range.

T800 SEMI-AREO
CAT 475
DIRECT 10 SPEED
Un sure of rears

Point of this is 2 different trucks, engines. Engines producing the same amount HP(assuming the det was turned down to the 475 rating full time) Other wise the DET just really sucks lol. And nearly Identical mpg.

I keep my speed down.. coast it up to 70 down hill. Don't idle more than I need to. Keep the turbo from spinning more then it has to. Progressive shift.

I'll be paying for this fuel someday.

Spec's and speed will determine your mpg. Simple, easy fact.

Good post, Serbie! 8)

It's also important, how you get to that "speed", be gentle with that pedal, it's not a NASCAR!
Keep it steady, remember,"up and down", cost you money!

I don't see anyone agreeing with you at all.

What I'm saying is you take any truck and slow down, the fuel mileage will increase.

Go faster and it will decrease, sure you can change the gears to get back into the "sweet spot" but you will still be down MPG.

You're getting a few things wrong here. First of all 80,000 lbs at 60+ mph is an absolute fuckload of momentum. Second, if there's next to no resistance, why does it take 200+ HP just to maintain speed? If there was no resistance(or very little) we would be getting 100+ mpg with our trucks at speed.

[quote="allan5oh"]I'm to tired to do the math... but that would be sweet... and getting a fuel surcharge to boot... holy hell lol :shock: :D :lol:

( I know this is impossible today.. but still fun to dream right.. eh.. huh?!?!?)

And that's exactly what I'm talking about. I see people @ my terminal.. bob tailing.. bringing the RPM'S to the limiter... THEY ARE IN THE YARD?!?!

I use just enough peddle to get the truck rolling.. when I can.. grab the next gear with lugging the hell out of the motor and do it again.. till I'm up to road speed.. then maintain that speed with as little peddle as possible.

Did allan post something???

It must be hard work to be so dumb allan but I admire your persistence. Typical super trucker who knows everything.... :roll:

There you go with your ad hom attacks again. Sorry, I'm not going to stoop to your level.

Again, I ask for you to participate in the debate, and counter my points. So far you've acted like a 13 year old kid.

anyways, until you bring up some valid points, I'm done here.

Quit crying allan and if you think I'm attacking you then that is sad. You tell me to offer valid points and to counter your points. Well you haven't made ANY, you just throw crap out in the air and hope some one catches it.

And it's about time your done here, it less garbage to sift through.

Don't waste your time Allan. I think most everyone on here agrees with you.

That's just sad if you guys agree him 100%. I said it before that in some applications slowing down does save fuel, but in others it does not. How hard is that to understand.... seriously.

and you completely failed to give a decent example, or even sound logic. You stated that you worked for a nascar team, and two different cars on two different tracks got the same fuel mileage. That doesn't even begin to apply to the real world.

If you think that a car/truck can get the same fuel mileage at 70 mph then at 60 mph, your a complete fucking moron. That's all there is to it. One truck, not two different trucks, ONE truck. Slow it down to 55 mph, and it will even get better fuel mileage. If it's geared incorrectly, you can drop it to 12th gear and still get better fuel mileage. I've tried this several times.

So what can you bring to the table other then your "stupid" comments? So far you've completely struck out.

Here's a link for you to chew on:

http://www.kenworth.com/brochures/FuelEfficiency.pdf

http://www.kenworth.com/FuelEconomyWhitePaper.pdf

Hmmm I have PERSONAL experience with getting better fuel mileage by going faster. Oh and here is the biggest thing in that brochure.....

So your saying that if in high gear my sweet spot is 65-68mph I should slow down and run below that to get the best fuel mileage??? In all that aerodynamic hog wash even they say to follow the ENGINE's manufactures recommendations for best fuel economy. So what now all knowing super trucker?

Ah hah! Sweet spot is NOT where you get the best fuel mileage.

The sweet spot is the best compromise between fuel mileage and pulling ability.

Also, the sweet spot changes depending on the load of the engine. Notice that engine manufacturers recommend a higher cruise rpm for heavy haul applications. Do you think it will get better fuel mileage? No it will not, it's a compromise between mileage and gradability. The same can be applied to speed. You'll need more rpms to run at 75 then you would at 55.

Show me anywhere that states running slightly below or below the sweet spot will result in worse fuel mileage. I'm begging you.

And yes, if you slowed down, you would get better fuel mileage. Try doing a few repeatable tests, eliminating variation (temperature, weight, wind, driving habits, overfilling the tanks, always filling up at the same spot). You'll be surprised.[/i]