This guy knows how to do it. He is also willing to drive slow enough so aerodynamics of his "brick" are not killing his fuel economy. Aerodynamics have far less impact at 55mph.

I'm seeing trucks start to slow down a bit.

There are still plenty cranking along working through traffic, going 70-75 mph. I wonder how many of the same trucks are then sitting and idlling later on.

I'm not telling anybody WHAT to do, just sharing HOW to get more out of a gallon. It's your money.

Noone has mentioned ratios ??? How much will a 3.25 differ from a 3.75 ???

You dont understand the workings of a bridge CAT, no matter what you do and how you drive it, it will get 4mpg stock.

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LOAD EM LIKE RAIL CARS, DRIVE EM LIKE RACE CARS!!!!

I am truck shopping and there is some good info on aero and fuel mileage in this thread, I think.

I have been wondering - where does all of the air that comes in through the radiator go out and does it create much drag as it exits the under hood area??? Anyone know if it would be more efficient and reduce drag if the air were allowed to escape through some louvers or vents toward the rear of the hood, either on top or on the sides? It might even be good for the running gear by reducing the temperature of the air flowing over the tranny, clutch, etc, also reducing underhood temps may be good for other components like air compressor, A/C compressor, alternator, etc?

Another thing I've wondered about is under truck drag. Is it reduced by a big 'ol bumper 3 inches off the ground? Any experience or thoughts on these wonderin's??? Thank you

I think it would be hard to do anything under the hood, alot of trucks barley have roon for the half of the engine that sits out there The big bumper if it did help would be a real pain, my 20' bumper sits about 8-9' off the ground and it is really hard to keep straight. got to watch for big holes in parking lots, concrete stops, and you definitley cannot fall off the road into a ditch backing in a driveway

The bumper can help by cleaning up the front and helping air pass under and around. True results are hard to predict though.

The test truck that was getting close to 11MPG had a lot of aero development. COE with full fairing, side skirts, full belly pan, smooth wheel covers etc...


If you are willing to drive 55-60 MPH the effects of Aerodynamic horse power loss are much less.

There is a common misconception that High horsepower engines use more fuel. They do not if operated well (at least not in a similar displacement engine)

It's all about horsepower demand. How much horsepower it takes to move the truck at a certain speed.

The High horse power engine should be the same in an identical truck cruising, but will often do better pulling hills with the higher torque. This is because of less downshifting and being able to pull the hill at a lower RPM (and pulling hills for a shorter time due to higher speed).

Good post. Agree with what has been said about aerodynamics. I get better fuel mileage running 80,000 lbs of steel plate than I do running 40,000lbs of insulation. I've often wondered what effect tarpped loads, especially on odd shaped loads where the tarps cannot be gotten tight against the load, have on fuel milage. If I have multiple dimension pieces I always try to get the higher pieces up front. Sometimes, due to weight constraints, it is necessary to have a big piece on the back, acting like a parachute. I read somewhere that after 30"" air space it was like having a whole new frontal area. I read an article on truck aerodynamics and it talked about vans and tankers. At the end of the article it said "We have not discussed open deck equipment because it is a lost cause aerodynamically speaking."


I agree with 24valve puller on cruise control. I can see the terrain and know what torque will be required. Cruise control does not know this. I've tried cruise control and turbo pressure goes out of sight on uphill pulls. It is like the cruise control wants to get to the top of the hill as fast as possible. With manual control I will drop down to 1000 RPM with little throttle and watching pyrometer and let the engine pull me to the top and let the engine slowly pick up speed at the top. I wonder if cruise control studies have been done on cars with different requiremants and not on trucks.

Thanks to everyone for their informative input.

There is a formula that you can use for this. I am working on getting one of my mechanics to use for these two ratios listed.

But I will try to explain as best I can until the scientific results are back. I will post those results as soon as he gets back to me on it.

I do know there is a 'sweet' spot when talking about trucks. In our trucks we have opted to use the 3.73's in most of them and that usually gets us anywhere from 6-7 mpg on average with sometimes running into the 9 range when loaded and driving at 68 mph.

But I think since the trucks are going to be pulling loads if you go to low in the ratio it will kill the gas mileage and if you go to high it will also kill the mpg you will get.

In theory though, the Higher the ratio the better the pull but it makes a dent in gas mileage. On the other hand, if you go to low with the ratio then it wont pull worth crap but might yield better gas mileage if you where not pulling/hauling anything and dont hit many hills, etc. Hense the sweet spot of lets say, 3.73 is kind of the mid range between the 4's and the lower 3's...........

Hope this helps, I think you will find the formula very cool so I will try to get that asap.

Best,
Derek

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Derek

You're probably not too far off there. The Western Star I spoke of earlier had 3.58s, 13 speed OD and 22.5 rubber.

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I suppose I should think of something to put here, huh?

Just got my truck back from the Cummins dealer and asked them to give me a printout of the ECM.

I have a 1996 Western Star 4964, Cummins N14 set to 460 hp, 10 speed trans, 3.70 rears, 11.00x24.5 tires, M&S on the rears.

Recent specs showed:

Average fuel economy: 5.50 mpg
Cruise Control Percent Time: 45.70 %
Distance: 4105.60 mi
Fuel Used: 745.0 gal
In Top Gear Percent Time: 89.99 %

I have progressive shift programmed into the ECM, altho I don't really use it. I try to up-shift around 1500 rpm and down-shift around 1150 or 1200 rpm. Cruising speed is usually between 1350 and 1500 rpm (from 62.5 to 68 mph or so).

I use cruise control mainly to avoid speed violations, but also to take some of the burden off of my brain. I will put my foot in it when I have to, especially when there is a group of turtles ahead and I have an opportunity to get past them and operate in an open space.

I am a firm believer that keeping a satisfactory distance between me and the next guy ahead of me allows me to back out of the accelerator (avoiding the service brakes) and also allows me to accelerate at a lighter pace. I do everything possible to avoid hitting my brakes.

The steadier the speed, the better the fuel economy.