[allan5oh]

Here's an excellent link:

http://www.everytime.cummins.com/every/pdf/4103999.pdf

page 4:

For maximum fuel economy OR for vehicles intending to cruise greater than 65 mph, gear for an engine speed of 1450 at 65 mph

look at that, they're recommending a different rpm for different speeds. Who woulda thunk!

In other words, it takes greater HP(and fuel) to go faster, therefore you need more rpms.

[/b]

[allan5oh]

and another one:

http://www.everytime.cummins.com/eve...Whitepaper.pdf

go to page 28, they state an incorrectly geared truck can reduce fuel mileage by 4%, yet speed is affected .1 mpg every mph above 55.

That means going from 55 to 75 mph affects your mileage by 2.0 mpg. Going from 6 mpg(average US truck) to 8 mpg is a change of 33%.

33% > 4%

[samael9]

No one has brought up the Turbo 3000D subject again once it was mentioned a few pages back. A friend of mine recently bought one and has been raving about its virtues. Anyone got any experience with this gizmo? I've always been skeptical of these kinds of "miracle" widgets...

The sweet spot that everyone is referring to has to do with the peak volumetric efficiency of a particular engine and its configuration. If a torque/horsepower graph were available for the engine in your truck, you would see where the best engine speed for efficiency lies. It is at the the engine speed where the horsepower and torque curves intersect.

Wind resistance is a big factor and, by fact of physics, does increase with rolling speed. The mileage numbers won't differ by a whole bunch until the truck leaves 55 mph and goes past 65 mph. Then, by virtue of drag, the mileage figures drop rather sharply for every additional 5 mph.

My W900L averages between 5.9 mpg and 6.8 mpg, with a record high of 7.2. I generally run @ 66 mph. The numbers went up by approximately 15% when I added the top splitter and converted to a 13 speed. I've run the truck more slowly, just to see if there was an appreciable gain in mpg, and frankly, it wasn't enough to justify the increased travel time.

There is a company that makes a low-restriction U-turn muffler which claims to increase exhaust efficiency by .5 mpg. As soon as I find the name, I'll be sure to post it.

[allan5oh]

BSFC (efficiency) usually is inverse of the torque curve(lower bsfc is better) in other words the engine is most efficient at max torque.

Problem is, max torque is usually about 500 rpms wide.

Another big problem is that I've NEVER been able to find BSFC numbers for any OTR trucks. I'd love to get my hands on BSFC charts!

[silvan]

Originally Posted by jegzus

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.

I should know better than to jump into this pointless fray, but I have to point out that if you're going to use that Kenworth pamphlet as proof of anything, you should read the bit at the bottom where it says "Slow down. Fuel economy is improved when drivers maintain 60 MPH as their cruise speed.**" The ** refers to a little tiny disclaimer advising drivers to obey the speed limit, implying it might be less than 60 in some places.

Personally, I'm willing to concede that slowing down delivers better fuel economy. However, I'm not willing to actually run 60 mph, because I would go nuts. I'd have a lot more money in my pocket if I only ate cheap food, sat on milk crates and construction lumber at home, got up two hours earlier so I could walk across town to where I pick up the truck, etc. It's all about compromise, and this is a trade I'm willing to make in order to make my life more enjoyable. Plus I don't pay for the fuel, and the O/O I drive for hasn't said boo about what kind of mileage I'm getting, so who cares.

Really, my only interest in fuel economy is that if it really is inefficient to run at 55 at the top of 9th gear, then Tennessee is full of :dung: about these no fly zones cutting pollution. Unfortunately, it doesn't seem to be the case, as much as I wish it were so.

You may resume your bickering. Sorry for the interruption. Have fun!

[BoyNextDoor]

Originally Posted by Rawlco

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.

Rawlco, sort of i have to disagree.

While i was with TMC, i had a 387. C15, 475/1175 torque(sp?) and usually drove 65 (limited) and got about 5.7-5.9 miles to the gallon as per the tmc certification reports. I once had a load that had to go 800 miles over the weekend (hence having a lot of time) i decided to run it at 55MPH. now before i started, i fueld up the truck all the way. load was heavy about 45k. When i reached my destination, i immediately fueled up again, and got an actual 6.87 MPGj that is fuel devided by milage. The onboard computer showed 7.02 MPG. I did the 55MPH in 13th gear, meaning that my rpm's were somewhere in the 1200's. It does make a difference.

Now that i'm drivin an O/O's truck i get about 5.7 MPG while being fully loaded, and driving 70-75. it's geared differently, has i believe the 3.77 rears, and 1850 torque but the same cat c15 475, and same tranny. This btw is also a 387. it does improve going slower. it all comes down to do you have time?

I had a load that had to be in SF bay area in 2.5 days starting from Cleveland. i did whatever was "safe" lol... and made it, with small amount of sleep, but avaraged about 4.8-5.2 mpg. have yet to do the math on that one...

i/m not saying that jegzus is wrong, my passat that i had was doing better mpg @ 80 than at 65. but ijn the truck world, where you have a lot more wight, and a lot more circumstances it just does matter...

[Graymist]

Originally Posted by silvan

You may resume your bickering. Sorry for the interruption. Have fun!

With all due respect to all parties in the fray here, I thought that last bit by Silvan was hilarious !!!! :lol: :lol: rofl