I'm driiving back from Texas to Ontario, Canada. The load is ... 6,000 lbs and I also drove empty for some 130 miles between Laredo, TX and Pharr, TX. I keep RPMs at 1300-1325. With the speed of 59 mph, my fuel mileage for the past 1000 mi has been 7.0 mpg. I agree with the flatbeds saving fuel - my 53' dry van trailer catches a lot of wind, even when I'm empty.

 

Pulling a tank at 79,000 running 62mph I can get 6.2 with my 379,better in the summer.

 

I think aerodynamic factor is overestimated in trucks. It's all in how close you stay to your "sweet spot" RPM. Classic trucks get more quality air because of outside air breathers, and according to Kenworth website these breathers only increase the annual fuel bill by 1%-2%.

 

Running 58mph? Yes. But try to run 75-80mph, you'll see the difference.

 

I think the opposite. Aerodynamics affect more MPG then the "sweet spot" ever will. I remember doing a test in North Dakota during stable winds. Going just 2-3 mph faster affected mpg much more then going from 1300 to 1900 rpms.

The horsepower required to maintain speed increases exponentially with speed. The most rpms will affect mpg is 3-4%.

All aerodynamic trucks grab outside air as well, at least the ones I've seen. Having the filter inside or outside won't change a thing.

 

Yes and no. The most efficient way to run a hill is to get to the top with as little shifting as possible while keeping the rpm within the fat part of the power curve, and then coast down to the bottom. If you are heavy, you may reach the top at let's say 40 mph having dropped one or two gears. If you allow gravity to accelerate you to the bottom and increase your speed, you are using ZERO fuel to accelerate and travel the distance down the hill. This is why I hate running I-5 in CA and OR. You may average 2.5 mpg going uphill, but then have to stay in a lower gear and / or get on the Jakes to control speed going downhill. You never get the chance to allow gravity to fully work for you. Running from Portland to LA grossing 80k, I will average maybe 5.8 mpg. Running that same load from Memphis to Raleigh, or Kingman to Dallas OK City I will average 6.5 to 6.8 mpg due to straighter downhill runs and higher speed limits. Your hypothesis is flawed in that the ideal would be to reach the bottom of hill going as fast as gravity will pull you and not brake at all. Of course this could result in a ticket, so let’s modify that statement to say that you should want to reach the bottom of the hill as fast as gravity will pull you and not be going so fast that you get a ticket. In addition, when the road has a close series of hills, you can benefit by doing the following: Let’s assume that you have your cruise set at 62mph and the speed limit is 70mph. You get up the first hill and coast down the back side reaching the bottom at 75mph. There is immediately another upgrade at the bottom of that hill, so you put your foot on the throttle at the beginning of the upgrade to use your built up momentum and speed (that was gained for free, due to gravity) to help you get up the next hill without having to downshift. This is what I do and the lifetime average mpg (actual, not ECM which is never accurate) of my truck is 6.8 mpg. If you have a CAT engine, you will find this same advice in the book on your engine in the “driving for good fuel economy section” of the book.

Also, more horsepower can result in better fuel economy if it allows you to climb a hill without downshifting. Your truck only generates the horsepower that it needs to maintain a given speed, so even if you have a 600hp engine, it will only burn enough fuel to create the ponies needed for the given speed, load and grade. Those extra horses are only generated when the conditions or driving habits or the driver demand that they be produced to make the engine perform as demanded.
Notwithstanding how a truck is spec’d, it always takes more fuel to drive and / or accelerate faster. Getting back to your physics, the power required to overcome air resistance increases roughly by the cube of the speed. The faster you drive the more fuel (by a power of three, or to be more precise, 2.6)it takes to overcome the drag. I know that some trucks because of how they are spec’d will be more efficient at 70 mph than they are at 60mph, but a truck properly spec’d to run at 60mph will always get better fuel economy than a truck spec’d to run 70, assuming that all other variables are constant.

 

If that is true, check your fuel economy driving into a 30mph headwind, and then turn around and see what it is running with a 30 mph tail wind. Driving into that 30mph headwind, I might get 5.5 mpg. Running with the tail wind I could get as much as 8 mpg. Take it from a physics phan, above 45mph, wind resistance is by far the largest factor in fuel consumption. If you don't want to take it from me, Google "wind resistance fuel economy" and see what others who are probably smarter than me have to say. Find one trustworthy source that agrees with you (other than longhood owner operators who are trying to make themselves feel better about driving a concrete block) and get back to me. Also, those breathers in the grand scheme of things are nothing when compared to the effect of drag on the entire truck at 70 mph. Also, 2% increase in fuel cost at $4 per gallon, 6 mpg, and 140k miles a year is almost $1900 a year, or nearly $10,000 over five years. I hope they're pretty, because those cans are going to be awfully expensive.

 

I drive at 58-59 mph in my aerodynamic truck. I don't do 140k a year but maybe 110k. the fuel i buy is $2.45 a gallon not $4/gal; I get 15 cents/mi back as fuel surcharge.

110,000 / 6 mpg = 18333 gal x 2.45 = $44,916

REBATE: 110,000 mi x 0.15 = $16,500

Annual actual cost of fuel: 44,916 - 16500 = $28,416.

2% mpg loss of $28,416 is $569/year or $47/mo; not $1,900/year.

Outside air breathers provide MORE air and this air is COLDER which actually improves fuel economy compared to trucks with air filters hidden near the engine block (check pittsburgpower.com). Classic trucks also command higher resale value and are easier to get rid of.

For people who drive SLOW, it doesn't matter that much what kind of truck you drive. I'd say the most important factor is the weight of the load, then - the truck spec's.

 

I remember chatting with Bruce about that, and I asked him if he had figures at the turbo inlet that showed he was correct. He didn't. It's simply opinion, not fact. Again, I don't see why having the filter inside the hood would make a difference. Both are grabbing the air from the outside.

 

In your experience, what RPM gives you best fuel mileage at cruise with the CAT when loaded? Thanks.