Getting Better Fuel Mileage (Trucks)

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Keith Morganstein
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Getting Better Fuel Mileage (Trucks)

Post by Keith Morganstein » Wed Jun 27, 2012 8:04 pm

Re post of an article I wrote and posted here. It was deleted when general discussion was purged a year or two ago.

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Getting Better Fuel Mileage

I've spent a lot of time with trucks, truck engines and fuel mileage. With high fuel prices and bogus devices on the market. I decide to write up an article.

(This is the truth, but it might not be what everyone wants to hear)

The #1 factor in getting better fuel mileage is to REDUCE HORSEPOWER DEMAND. This is a combination of vehicle configuration, condition, load and DRIVER OPERATION.

The #2 factor in getting better fuel mileage is to AVOID RUNNING THE ENGINE WHEN YOU ARE NOT MOVING.


The #3 factor in getting better fuel mileage is the engine.
Everyone wants to look at the engine first , because they think the part that burns the fuel is the most responsible for fuel economy. I'll discuss this first and then get to the other issues.

Most modern on highway diesel are capable of BSFC's in the low .300's or better (high .200's) at cruise rpm. This is pretty #### lean compared to earlier engines and I haven't seen any after-market accessories offer an improvement.

So, as far as the engine, you just need to make sure the engine is mechanically/electronically correct to start.

Check air and fuel filters.

Inspect air intake and exhaust systems. Look for leaks in the after-cooler piping, after-cooler core etc... Best way is to plug at each end of after cooler piping and pressurize entire assy to test for leaks.

Check exhaust for blown manifold and turbo gaskets. look at manifold and turbo hot housing for signs of overheating. A visual of the cold side is helpful, looking for damaged fins, rubbing etc...

If mechanical engine, check timing. Also check linkage to pump. Check fuel settings if seals or broken, Check air / fuel ratio controls, (aneroid, puff limiter etc..)

If electronic, check for fault codes and possibly do a timing and sensor calibration.

Check fuel pressure.

Make sure engine is up to operating temp. Usually 190°

Check fuel quality: Fuel should be 35 API at 60°. Thinner winter blend will reduce fuel mileage considerably.

Fuel heaters: should be thermostatically controlled. Off in summer and only on when needed in the winter.

Oil: use 10w30 in all but the hottest weather. If you are running 15w40 in the winter it's hurting fuel mileage. You will see 4-5psi lower oil pressure, but it won't hurt anything.

Clutch fan: must cycle on and off. Fans require 15-18hp to run.

BLACK SMOKE: There should be no black smoke when changing gears or accelerating. Black smoke means the engine / fuel system is out of tune and adjustment for fuel economy. BLACK SMOKE is MONEY going out the exhaust pipe.

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VEHICLE: many considerations here. Start with tires. make sure they are properly inflated (wish i had $$$ for every tractor that showed up for a dyno that had low tire pressure)

Tread design: rib is better than on/off road and heavy lug tread is terrible, run rib tires when possible. Low profile has much lower rolling resistance too.

Drive line: check drive-shafts for misalignment's and vibrations. Rears must track straight, worn misaligned rear suspensions can cost you. An eyeball along the edge of the rears to the front can give you a quick check here. Use fuel saving synthetics in trans and differentials.

Vehicle aerodynamics / configuration: This varies greatly with different type of power units and trailers. The best possible is a Cab-over-engine (COE) with roof fairing, side fairings and very few protruding accessories, pulling a smooth side van trailer. Everything having ribbed low pro tires

The worst is a big conventional with no fairings, big external air cleaners , big mirrors, horns, fuel tanks, visors, no fairing, pulling a flatbed with a big bulky load, with lugged tall tires.

HORSEPOWER DEMAND: The horsepower that is needed to maintain a certain speed. i.e. how much horse power needed to overcome wind and rolling resistance, determines the fuel you will burn to cruise down the highway. This is called HORSEPOWER DEMAND. It also determines how much RESERVE HORSEPOWER is available for pulling hills and passing other trucks etc...

EXAMPLE: If two trucks have 400hp and truck #1 requires 160hp to maintain 60 mph and truck #2 requires 220 hp to maintain 60 mph, truck #1 will of course have better fuel economy AND have 60 more RESERVE HORSEPOWER to pull hills.

So aerodynamics play a huge role in fuel economy, particularly at speeds above 55MPH
(55 wasn't a arbitrary number when it was the federal speed limit)

Above 55MPH "AERODYNAMIC HORSEPOWER DEMAND INCREASES BY THE CUBE OF SPEED INCREASE".
How this plays out is a truck that can achieve 7 MPG at 55 MPH, will get 6 MPG at 65 MPH and only 5MPG at 70MPH

Aerodynamic considerations include having a roof fairing that matches the height of the trailer. The trailer should be as close to the tractor as possible. Ideal distance is 15-18 inches, never more than 24 inches. Side fairings help close this gap without the trailer being to close for sharp turns.
At distances over 24" inches between the trailer and the tractor, drag goes up considerable. by 48" you have the drag of TWO FULL FRONTAL AREAS. Might as well have the trailer beside the truck! So much for the stretched out "west coast" look.

It can be tough to improve an existing tractors aerodynamics, but you can add a roof fairing and side fairings. Avoid putting on a lot of bulky accessories like visors, bigger mirrors, dual external air cleaner, huge bumpers, horns, flaps hanging everywhere, etc... Remember, driving an un-aerodynamic rig fast, will hurt fuel mileage even more.

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DRIVER OPERATION: This is the big one that no one wants to face up to. However there is a BRAND NEW PICK-UP TRUCKS WORTH OF FUEL SAVING$ PER YEAR AVAILABLE (to those that operate well VS those that don't)

#1 Cruise speed: I just went over aerodynamics and horsepower demand.
More time at lower speed will save you big on fuel.

#2 Cruise RPM: most modern trucks will operate efficiently in the 1400-1600 rpm range depending on model of engine. Cruise at this rpm (in the lower range of it if possible) Do not gear down a step to run at 1800rpm or higher

USE CRUISE CONTROL, many electronic engines alter timing maps in cruise control for better steady state economy

3# PULLING HILLS: Diesel have a fuel curve inverse to the torque curve. (on some electronic engines this flattens out)

A typical 400 hp engine may burn 18 gallons per hour (GPH) at 1800 RPM, But only 14 GPH at 1200 RPM. Meanwhile it has MUCH MORE TORQUE AT 1200 rpm.

What does this mean? You generally pull hills at max fuel. You want to pull hills in the highest gear you can and keep the engine at peak torque/ lower GPH.

A big mistake is to drop a gear and run at the hill at higher rpm where the engine uses more fuel and has less torque. This is a fuel wasting downward spiral were you have to make successive gear changes burning more fuel and going slower and slower.

Stay in the highest gear as long as possible in hills, if you can crest the hill at or above 1000rpm, it's usually o.k. to do so.
Think about the savings in hilly terrain, hill after hill BURNING 14GPH vs 18GPH to get the job done. Add that you are usually pulling at a higher speed with correct method so you spend less time at max fuel consumption.

#4 GEAR CHANGES: Shift progressively, use only enough RPM to get into the next gear. Taching out every gear is a waste of fuel, especially in city driving

#5 COOL DOWN: Use exit ramps and truck stop access roads to cool your engine down before shutting it off. If you can coast in to a stop using very little fuel and keeping the exhaust temp down, there is no need to let the truck idle several minutes before shutting it off. Idling is a big waste if fuel and five minutes has a way of turning into twenty five.

#6 IDLE TIME : THE BIGGEST WASTE OF FUEL OF ALL. Any time that engine is running and the truck isn't moving hurts tank mileage and your wallet. It is also unnecessary engine wear. Shut the truck off whenever you can. Cool engine down as outlined above. Consider alternate heat/cooling options for over the road trucks. These are best installed when the truck is new for maximum payback.
An electric blanket or good sleeping bag can get you a few more comfortable hours in sleeper without having the truck running.

#7 DRIVE SLOWER (did I say that yet?)

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Re: Getting Better Fuel Mileage (Trucks)

Post by raceman14 » Thu Jun 28, 2012 10:48 am

Keith,
Awesome post, back in the day when I built spoked race wheels for Bicycles, I used to try and run maximum tension on all of my spokes and with tubular silk tires we could race at 150-160psi. If you ran to hard or too hot the resulting explosion sounded like a 30-06 going off and your bike and you would raise up about 2' off the ground. In most cases you could save it and coast to a stop, in rare cases a rear wheel would collapse under the de-compression load and that made a pretty big mess of things.

Anyway my point to that was I try and run maximum pressure that is driveable and comfortable. My 99 suburban can go from 15.5 mpg to 17mpg going from 36 psi to 50 psi. Just did it again on a trip to Destin. I do drop back down for local driving as the ride and handling are much better at 36.

On the chassis dyno, I have seen power increases of 20-30hp with increased tire pressures. Even more with decrease in Stagger. Most racers don't have a clue or know where to find it when it comes to making more power by changing minor things on their race cars. I guess having a Chassis DYno for 25+ years changes the way you think...

No let me change that, working with Bill Jones and then buying a ChassisDYno because you want to know HOW and WHY stuff makes a difference, changes how you think...
More is always better!!! Most of the time.

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Re: Getting Better Fuel Mileage (Trucks)

Post by PackardV8 » Thu Jun 28, 2012 1:02 pm

Yes, tire pressure makes a real difference. On my wife's new SUV, at a cruise controlled 78 MPH, the recommended 34 PSI tire pressure = 21 MPG. Going to 39 PSI = 24 MPG on the same stretch of I-84.

This is the first non-turbo vehicle we've owned in many years. It takes some getting used to having the six-speed auto drop out of locked converter, then down a gear, then down another gear, turning 4200 RPMs to hold 78 MPH up a mountain. On the same grade the turbo would just ramp up the boost and maybe unlock the converter. The FWD turbo 4-cyl sedan will get 33 MPG where the VVT direct injection 6-cyl SUV XWD does 25 at best.

If lots of miles are involved choosing a FWD over a 4wd is a no-brainer. Having the drag of the all-wheel geartrain, plus the oversize tires always included today are a sucking chest wound. We drove FWD sedans with top quality snow tires for forty years and always got where we were going in Montana in the winter. It will be interesting to compare this first XWD. It has a unique 6 X 120mm lug pattern, so finding a second set of wheels for snow tires is proving difficult.

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Re: Getting Better Fuel Mileage (Trucks)

Post by raceman14 » Thu Jun 28, 2012 2:16 pm

You might have to opt for a set from the local scrap yard if they have anything that new or an aftermarket set of cheap aluminum wheels if they are even offered.

There is a guy on e-bay making some real nice CNC adapters but they are pretty expensive on their own.
More is always better!!! Most of the time.

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Re: Getting Better Fuel Mileage (Trucks)

Post by crazyman » Thu Jun 28, 2012 3:19 pm

PackardV8 wrote:It has a unique 6 X 120mm lug pattern, so finding a second set of wheels for snow tires is proving difficult.
Dodge Dakotas are 6x114.3, so they are out of the question as are Nissan Pathfinders at 6x139.7.

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Re: Getting Better Fuel Mileage (Trucks)

Post by Mike Croley » Fri Jun 29, 2012 3:31 pm

That's an excellent post Keith . Very well thought out .
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Re: Getting Better Fuel Mileage (Trucks)

Post by JBrady » Sat Jul 07, 2012 2:06 pm

Great post. Same concept applies to maximum performance. So many guys want more power when they are not making full use of the power they already have. Getting the most out of any combo takes a thorough careful effort but worth it on many levels. Not to mention the bragging rights of getting the best economy and/or beating a "faster" car/truck/bike/boat/plane...

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Re: Getting Better Fuel Mileage (Trucks)

Post by roadrunner » Mon Aug 27, 2012 6:47 pm

Interesting post! Have you guys tried (with success) any friction modifying additives for the engine/drivetrain?
Life's too short to not run a supercharger!

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Re: Getting Better Fuel Mileage (Trucks)

Post by Warpspeed » Sun Sep 23, 2012 11:51 pm

This really applies to any vehicle, not just trucks.

One very useful instrument would be an instantaneous fuel usage readout, it would be a great learning tool for selecting the right gear, and learning a better general driving technique.
It would also show up any small improvements such as reducing rolling resistance, improved aerodynamic drag, and reduced vehicle weight.

There can also be gains from fitting more suitable diff gearing for the terrain and type of driving.
For example, flat/hilly or city/freeway. A shorter or taller final drive can sometimes give quite unexpected results either way. But you never know until you try.
Cheers, Tony.

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Re: Getting Better Fuel Mileage (Trucks)

Post by jim_ss409 » Thu Oct 04, 2012 11:31 pm

If I had a nickle for every trucker that told me that HIS TRUCK gets better fuel mileage at 75 mph than it does at 60 mph, I'd be rich. The worst part is that I often hear this from guys that have very little in the way of aerodynamic aids. Of course they're always quick to add "My truck is geared to run 75 mph"
Sure, if you are going to 75 mph it's best to gear the truck accordingly but no magic gear ratio will overcome the extra drag.

Another bad habit I see is not managing the trucks momentum well. There is a lot of fuel to be saved by cutting the throttle sooner and coasting longer when you're taking an off ramp or stopping at a toll plaza. Many truck drivers hit the off ramp at full speed then brake hard while downshiftingaggressivelyvely as they can.

Most drivers these days drive with the engine brake in the on position all the time. So there is no real coasting.

A good truck driver that puts his mind to saving fuel can easily beat an average driver's fuel economy by 10%. A smart owner-operator that specs, maintains and drives a truck the way Keith described can easily save 15 or 20% over the average truck on a similar haul.

My newest trucks have this Volvo XE13 package. They have automatic transmissions and run at only 1,100 rpm @ 60 mph. They are very efficient and somewhat idiot proof yet my best drivers always get about 5 to 10% better fuel economy than the worst drivers.
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Re: Getting Better Fuel Mileage (Trucks)

Post by PackardV8 » Fri Oct 05, 2012 1:39 am

They are very efficient and somewhat idiot proof yet my best drivers always get about 5 to 10% better fuel economy than the worst drivers.
FWIW, I once managed a fleet of gas burners and the spread between the best and worst was 50%. It would have been more, but there was a lot of city driving and the real idiots had to be at idle a lot of the time. Otherwise, they were at full throttle. There was also a direct correlation with brake wear.

I asked to have drivers rewarded for better fuel mileage, but the powers that be didn't want the extra paperwork and complaints. Have any commercial fleets given drivers an incentive to conserve?

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Re: Getting Better Fuel Mileage (Trucks)

Post by jim_ss409 » Sat Oct 06, 2012 12:20 am

PackardV8 wrote: Have any commercial fleets given drivers an incentive to conserve?

jack vines
Yes, some do but it can get complicated because some trucks will get heavier loads than others or get trips with more hills etc. I really should be doing it. The efficient drivers should be getting some kind of financial reward for saving fuel.
Some trucks can be programed to allow the drivers to run quicker if they're meeting the fuel economy target. I don't use that option but my trucks are governed at 65 mph with a maximum cruise control setting of 62 mph.

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Re: Getting Better Fuel Mileage (Trucks)

Post by Kevin Johnson » Sat Oct 06, 2012 8:49 am

jim_ss409 wrote:
PackardV8 wrote: Have any commercial fleets given drivers an incentive to conserve?

jack vines
Yes, some do but it can get complicated because some trucks will get heavier loads than others or get trips with more hills etc. I really should be doing it. The efficient drivers should be getting some kind of financial reward for saving fuel.
Some trucks can be programed to allow the drivers to run quicker if they're meeting the fuel economy target. I don't use that option but my trucks are governed at 65 mph with a maximum cruise control setting of 62 mph.
Have you evaluated products like fuel deaerators and hydrogen boosters?

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Re: Getting Better Fuel Mileage (Trucks)

Post by jim_ss409 » Mon Oct 08, 2012 12:03 am

Kevin Johnson wrote:Have you evaluated products like fuel deaerators and hydrogen boosters?
No I can't say I have. I've skipped through some web sites for fuel enhancement devices but the "proof" that the products work as advertised is usually just testimonials rather than SAE style testing. I don't know, some products may work but I'm not seeing a lot of convincing evidence.

I have recently converted from 15/40 oil to 10/30 oil. I think this may help a little, especially in the winter months. I noticed that Delo oils from Chevron has begun to market synthetic motor oil for fleets in a serious way. The feeling in the trucking industry has been that synthetic gear oils are certainly worthwhile but synthetic motor oil wasn't worth the extra cost. With the rising cost of fuel, maybe synthetic oil is becoming more cost effective, especially in the winter months.

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Re: Getting Better Fuel Mileage (Trucks)

Post by Kevin Johnson » Mon Oct 08, 2012 6:37 am

jim_ss409 wrote:
Kevin Johnson wrote:Have you evaluated products like fuel deaerators and hydrogen boosters?
No I can't say I have. I've skipped through some web sites for fuel enhancement devices but the "proof" that the products work as advertised is usually just testimonials rather than SAE style testing. I don't know, some products may work but I'm not seeing a lot of convincing evidence.
I ask because on another forum a mechanic crawled under a fleet truck and snapped pics of an expensive booster which I suspect was installed to evaulate claims. I would repost them but I got hit with a copyright takedown notice when I did so previously. Apparently the mechanic was in fear of loosing his job despite having posted the pics in the first place. (?) This is going back several years now.

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