Facts that affect MPG

Choosing the specs for your truck can seem complicated, especially when listening to everybody else’s opinions. Everybody has a different view and only you can know exactly what you need in your operation.

Whether you are buying new or used the specs can make a big difference. Transmission and rear end combinations alone can cost you over a mile per gallon depending on if it is set up for regional, heavy haul or long haul applications.

Utilize the internet to arm yourself with knowledge. Cummins, for example does a nice job of providing useful info on specs in their papers. Here is one worth checking out before looking at purchasing another truck, new or used. “10 Tips to Maximize Fuel Economy”

Your need for speed may change from day to day and load to load in some operations. Slowing down when time permits is one of the most cost effective ways to save fuel. Generally speaking, every 1 mph over 55 mph will reduce your mpg by .1 mpg.

But, time is money! So, if by shifting into overdrive and running 70 mph makes it possible for me to get delivered early Friday instead of late Friday I’ll spend the extra on fuel, grab another load and be able to run the weekend instead of sitting all day Saturday and Sunday.Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.

What is the sweet spot we all here so much about? RPM? Speed? MPG? Or is it the money in your pocket at the end of the day? And how do we find it?

The sweet spot that most talk about is where we feel our engine likes to run, feels responsive, not over-loaded or lugging, just easing down the road without a need to shift on every little grade. Because the “Road Load” changes with each load your sweet spot will too. Finding that spot and maintaining it is important to eliminate over fueling.

That being said, it is important to get your gearing and transmission set up so you can take advantage of a light load in a good tail wind while being geared so you can also handle the heavy loads and head winds. I like to have the ability to run from 55 to 75 mph using the top 2 gears and stay in an rpm range between 1275 rpm and 1475 rpm. If I can keep my Cummins in the lower half of that range (1275 – 1375) it will still pull the small grades without shifting and fuel consumption is better. When my road load is heavy or I just have the time I’ll drop it into direct and run 55 to 65 mph. When I’m light, dead heading or have a good tail wind I’ll grab over drive and run as fast as the law will allow. For more details on gearing for the sweet spot see “Gearing Facts” below.

Sometimes there’s a bit of a gray area and it’s hard to tell if you should run higher rpm’s in direct or try to run in overdrive while maintaining the same throttle position. In most cases you’ll gain about .3 mpg if you can run in overdrive but you should keep an eye on your exhaust gas temps. You’ll be moving less air and even though you’re burning less fuel per mile you’re egt’s will run hotter.

Because wind resistance increases exponentially with speed the extra power and fuel needed to maintain the higher speed makes it much more challenging to find the most economical speed to run. This is why aero-dynamic trucks, along with trailer improvements are so beneficial. It would be much easier to find the sweet spot if wind resistance did not increase exponentially with increased speed. Running faster would always be more efficient. For instance, with your fuel consumption at a constant 10 gallons per hour, if you run 60 mph your mpg is 6, but at 70 mph it is 7 mpg.

Finding your sweet spot should be based on maximizing your time based on the revenue you can generate with your equipment.

OK, so here is the kicker! The true sweet spot on every truck will have you running 45 to 50 mph. That is where fuel economy and drivability are the highest. We all know that is not practical because time is money. We can buy more fuel but we can’t buy more time.

Example: If you are getting paid $4 per mile and you can run an extra 100 miles per day by running 10 mph faster you will gross an extra $400 per day. If your cost per mile is $1 you clear $300 extra per day. However, if the load is paying $1.20 per mile you will only clear an extra $20 per day, probably not worth the extra wear and tear on the truck. This is especially true if running faster is not going to net you more loaded miles at the end of the month.

The sweetest spot for me is the spot where my income is maximized. To find your sweet spot based on your revenue potential, check out the “Speed Efficiency Calculator” in the “Spreadsheets and Calculators” below. It will show you the effects of your speed vs. your expense. This spreadsheet uses the industry standard for wind resistance of 10 mph = 1 mpg for speeds over 55 mph.

I run over-the-road so my favorite setup is a 13 speed with 2.64 rears and the X-One wide base Michelins tires (with 515 revolutions per mile). With this setup the split is small enough that you will be able to keep it in a 200 rpm range. You will be able to run in 11th gear direct from 55 mph to 65 mph and the rpm range would stay between 1275 and 1475. Grab 12th gear and run the same rpms from 65 to 75 mph. The up side to this setup is you will be in direct from 55 to 65. The down side is you will need to move the shift lever each time between 11th and 12th gears. If you run heavy and you can limit your speed to 65 or less most of the time this is definitely the way to go. 70 mph in direct is about 1600 rpm.

Why do you want to run in Direct? According to a Cummins paper here, running a direct drive transmission vs an overdrive transmission will save you 3% in fuel economy. This is only true however, if you never plan to take advantage of a tail wind or a light load situation. If there is truly a 3% savings in fuel economy, that figures out to about two tenth of a mile per gallon. Having the option to gear up will save you 3 tenths, but only when the opportunity arises during light road load situations.

The other option is to run 3.08 gears and just use the splitter, but you will always be in an overdrive gear and will lose a bit of efficiency in the transmission by never using the direct gear. If you spend most of your time around 60 mph I would go with the 2.64 gear for a bit more efficiency but if you never run less than 65 or 70 you will be in overdrive anyway and probably splitting gears more so the 3.08s would be easier to drive.

Check out the “Axle Ratio” spreadsheet in “Spreadsheets an Calculators” below to find the setup that best fits your operation.

Fact: A gallon of today’s ULSD has lower energy content than the low sulfur diesel we used a few years ago which leads to a 1 to 2% lower fuel economy. And the lower energy content of Biodiesel blends will cost you 1 to 2% over using USLD. So, today’s fuel will cost you 2 to 4% in fuel mileage as compared to the fuel you bought a few years back.

Note: If your ECM’s mpg and fuel consumption readout is lower than your calculated fuel mpg at the pump your injectors could be dumping more fuel than they are supposed to. That is, if you have the correct set of parameters and OEM programing in the ECM. The ECM calculates fuel usage by counting the injector pulses, not by measuring the fuel volume so it will not be accurate if the injectors are over-fueling or leaking.

Fuel Filters

Aerodynamics will save more while running at higher speeds, above 50 mph.

Lowering rolling resistance is more important in operations that run slower speeds averaging below 50 mph.

Aero Body Styles –

Wheel Covers –

Tractor Skirts – and FlowBelows

Tractor Trailer Air Gap -

Trailer Skirts –

Trailer Tails –

Custom Aero -

Lowering rolling resistance is more important in operations that run slower speeds averaging below 50 mph. Aerodynamics will save more at higher speeds.

Worn tires provide better fuel economy than new tires, up to 7% better fuel economy.

Used lug drive tires can get up to 0.4 mpg better than new lug tires.

Ribbed tires on the drive axles provide 2–4% better fuel economy than lugged tires.

Every 10 psi that a truck’s tires are underinflated reduces fuel economy by 1%.

The break-in period for tires is between 35,000 and 50,000 miles.

Every 1,000 lbs. = a .5% increase in fuel consumption in over-the-road tractor trailers

EPA SmartWay presentations and fact sheets states that there is a .4 - 1% fuel savings per 1000 lb weight reduction.

It is important to point out that the heavy duty on-highway diesels produced in “1999” had a fuel consumption rate per horse power produced, or BSFC (lbs/BHPH) of .29 lbs. and by 2004 fuel consumption had risen to .33 lbs. per hp per hr. That’s a 13% increase in fuel consumption in a 5 year period to improve NOx levels which were reduced by almost 6 g/hp-hr.

That is a savings of 126 grams per gallon at 21 hp/hr.

So you are losing between 2 and 3 tenths mpg, depending on the speed you drive. Even with the technological advances in engine accessories such as fans, water pump, turbos, compressors, etc. the tighter emissions regulations have made it difficult for newer engines to match the efficiency of pre-emissions engines.

An engine’s total fuel consumed determines the life of the engine. Fuel heat energy determines wear.

BSFC (brake specific fuel consumption): There is a very definite relationship between the amount of fuel burned and the amount of horsepower produced by your engine. This is an important concept to understand, as discussions center around various competitive diesel engines and performance. From the best to the worst, none of these engines share more than a 4-5% overall advantage when comparing actual BSFC and would not stand up to the actual scrutiny of the datasheets on file with the EPA.

All properly tuned over the road diesel trucks in the last 20 years or so will produce around 20 hp per gal per hr.

Engine Size

Smaller engines can improve efficiency slightly because of lower weight and less friction as long as the duty cycle allows for the lower power.

However, I believe that if the larger engines are driven right they will last longer and be just as efficient as the smaller engine. I’m talking 11 liter vs. 15 liter, not hp ratings. Both engines will produce nearly identical power given the same fuel input. An engine that is programed for 300 hp will consume about 15 gallons per hour regardless of its size. The same engines programed to 500 hp will suck up 25 gph.

It is a common belief that a smaller engine is more efficient. This misconception is probably due to the fact that they are usually programed at lower hp ratings. A lower hp rating is nothing more than an automatic power limiter. Simply limiting the position of your right foot on a higher hp engine will do the same thing. The truth is there is still only 390,000 btu of energy in a gallon of diesel. You will only get out what you put in.

An engine cylinder with a longer stroke-to-bore ratio will have a smaller surface area exposed to the combustion chamber gasses compared to a cylinder with shorter stroke-to-bore ratio in engine with the same displacement. The smaller area leads directly to a reduction of in-cylinder heat transfer, increased energy transfer to the crankshaft and, therefore, higher efficiency.

For engine size comparison purposes, consider the following:

A 10,000 + hp ocean liner turns about 120 rpms and has a 8 ft. stroke and a 4 ft bore or a stroke to bore ratio of 2 to 1 and is one of the most efficient ways to move freight in the world.

Most large diesel truck engines (11 to 16 liters) use a stroke to bore ratio slightly longer than 1:1 and produce 20+ hp per gallon per hr.

A carbureted gasoline engine like your typical 454 Chevy will deliver LESS than 12 horse power per gallon per hour of gasoline consumed under the best of conditions.

A two-stroke outboard engine may give you 6-8 hp/gal/hr.

A two-stroke high output motocross bike is lucky to produce 4-5 hp/gal/hr.

Your Cox .049 model airplane engine is off the scale.

You get the picture.

ECMs

Turbos

Exhaust

Micro blue

Manifolds

Oil

Filtration, Air and Oil

Gearing and rears

Transmission

6 x 2

Lift axles

Most companies selling Hydrogen injection units are still in the development stages and have NOT been proven in the real world. I have worked in Research and Development and tested several different systems. The truth is these units are really experimental and most of the units are laden with problems. They have been on the market for well over 10 years and if they were as great as some of the advertising they would have been an industry standard like trailer skirt by now.

If you are knowledgeable and understand the Technology and want to pursue using Hydrogen technologies stick with a P.E.M. Cell technology using the Proton Exchange Membrane. There is one company claiming a modest 6 to 10% fuel savings in 11 and 12 liter diesels.

From my personal experience most units on the market are producing only enough hydrogen for a diesel engine of less than 7 liters. It is my believe a unit producing close to 1 liter of hydrogen per minute for every liter of engine displacement could enhance performance and mpg but I have not been able to confirm that.

Caution:

Most systems are chemical based to enhance the conductivity in the hydrogen cells. I have personally seen the use of these chemicals destroy turbos and charge air coolers.

How many hours of idle time does it take to pay for an APU? In my operation for example, it was wiser to invest in other fuel saving devises first.

Weed burners

Mufflers

The mpg comparison for “% increase in fuel economy” versus the “% decrease in fuel consumed” is non-linear.

% Increase in Fuel Economy	% Decrease in Fuel Consumption
10	9.1
50	33.3
100	50

Improving a 4 mpg truck by 100% = is a 4 mpg increase while decreasing an 8 mpg truck by 4 mpg is 50% decrease.