____Most Efficient Diesel_____


Dr. Jim Freim


Copyright ©2015 All Rights Reserved

I love diesels. I’ve lost count of how many I’ve owned. And I’ve driven them LOTS of miles. Around 800,000 – give or take a few! My moniker on automotive websites is Millionmiler. For this article I’ve included a1996 Dodge Ram 2500, 2003 Dodge Sprinter 2500, 2005 Jeep Liberty CRD, 1983 Mercedes 240D, and 1999 VW TDI. Comparing such diverse vehicles with completely different purposes may be a stretch, but in the back of my mind, I wondered, “Which diesel is most efficient?”

Diesels are inherently more energy efficient; they get better fuel economy (MPG) than gas engines. And they last forever. Well, almost forever. Over the road (OTR) rigs use diesel engines and may run 10,000 or miles per month. In one year, that diesel engine may have over 125,000 miles. The OTR engines may get overhauled at 1,000,000 miles, 8 to 9 years of driving. Gasoline engines in big rigs wouldn’t make sense. MPG would be worse, (guesstimate) at 20 to 25 % less, and the engines, because of the heavy hauling duty, might need to be rebuild every 300,000 miles (also a guesstimate).

For personal hauling / towing, trucks such as the Ford Powerstroke, Dodge Cummins, and Chevrolet Duramax will run many, many miles. I often see Cummins diesels for sale that have over 500,000 miles! One owner has over 1,600,000 on his Cummins. No rebuild yet.


Having owed several different makes and models of diesels, I wondered what was the most efficient diesel that I’ve owned. A partial list of the diesels I have owned is shown in the table. They very greatly in size, weight, and engine size. Some are 2WD, some are 4WD. Some have automatic transmissions, but three have manual trannys (my personal choice).

Vehicle Best MPG Engine Size Weight (1) 4×4 Automatic or Manual Transmission Aerodynamic?

96 Dodge Cummins






No, No Like pushing a piece of plywood through the air

99 VW Beetle






Yes, very

83 Mercedes Benz 240D







03 Dodge Sprinter Van






Somewhat with sloped nose

06 Jeep Liberty CRD






Like half piece of plywood

The 1996 Dodge Cummins is a 12 valve engine, quad cab, long bed, 4×4 with a manual transmission. The old 12 valves are considered to be the most efficient Cummins. These engines were made from 1989 to 1988. The truck is a lovable beast. In addition to hauling building supplies, water tanks, I must admit I’ve used it for pulling tree stumps. Not the best idea, but 4 low in 4 wheel drive will “get ‘er dun.” I routinely get 23, sometimes 24 MPG.

Illustration 1: 1996 Dodge Cummins 4×4


Illustration 2: 1999 VW New Beetle TDI


The VW Beetle is like an egg on roller skates, 2 doors, manual transmission. The engine is the 1.9 liter TDI (Turbo Direct Injection) These are tough little engines with lots of torque. In Europe, a TDI racing circuit exists. They flog these engines to near death, but somehow they keep ticking. (Reminds me of the old Timex commercials with John Camren Swayze if you are old enough to remember – if you are not old enough, check it out.) Easy driving netted 48 to 50 MPG

The 1983 Mercedes Benz is a quality car with a diesel engine that will run 500,000 miles. The car feels solid and the doors still close with a reassuring thud. The car is from sunny San Diego and lived a rust free life. In Colorado, I drive it in the summer. The engine is a non turbo and driving at Colorado elevations and mountains gives new meaning to enjoy the scenery! Driving from CA to CO I averaged almost 35 MPG.

Illustration 3: 1983 Mercedes 240D


Illustration 4: 2003 Dodge Sprinter with 2.7 liter Mercedes Diesel Engine

The 03 Sprinter Van is the 2nd Sprinter van that I have owned. This 03 was used as a delivery vehicle by a Dodge dealer. The van now has 240,000 miles and still performs well. A little ragged around the edges, but mechanically superb. Even though the van is equipped with traction control, uphill on gravel roads can be a challenge. When it snows, use another vehicle! These vans have been sold in Europe for many years and come in 4×4 and with manual transmissions. In the USA we only get the 2 wheel drive version with an automatic tranny. At 50 to 55 mph on back roads, 30 MPG is possible. Fully loaded and driving on the interstate, 22 MPG is possible.

Illustration 5: 2006 Jeep Liberty 4×4 with the VM diesel engine


The 06 Jeep Liberty is rare desirable creature only available for 2 years. The CRD (Common Rail Diesel) is made by VM Motori of Italy – they have been making diesel engines for over 60 years. Contrary to popular belief the engine is not made by Mercedes. I love this Jeep – good ground clearance for off road capability. Comes with hi range 4×4, a low range 4×4, and a partial 4×4 mode where a computer decides if 4×4 is needed – primarily for driving in inclement conditions. But missing is a manual transmission – not available with the diesel but available with the 3.7 gas version. The engine at 21,000 is still breaking in, but I got almost 30 MPG on a round trip from CO to WI, driving it “very” gently, i.e. like a ‘grandma.; For that size engine in a fairly small vehicle, I think the fuel economy should be better.


When I need to get somewhere, my foot is in it. “Pedal to the metal.” But, most of the time, I’m a leisurely driver. “What me worry?” I’ve eliminated the stress from driving too fast, too crazy, weaving in and out of traffic to save a few minutes, and worrying about the location of the gendarme.

I like driving the back roads of America. Slow down and smell the roses, meet the people, sample the small town fare, camp in out of the way spots. I’ve driven many miles in each of these vehicles – in the same manner. Easy acceleration – pretend an egg is between your foot and the accelerator pedal. If press too hard, the egg breaks – What a mess to clean up! Back road cruising at 55, coast up to stops. I wouldn’t call myself a hypermiler but I do use some of those techniques. Proper maintenance, correct tire inflation, and regular oil changes all help.

The table shows the MPG achieved with these driving techniques. If we compare just the raw numbers, the best MPG is the 48 to sometimes 50 in the 99 Beetle. The worst is the 23 to 24 in the 96 Cummins. But the 5.9 liter Cummins is the largest engine and the Dodge truck is the heaviest by a wide margin. The VW has the smallest engine (1.9 liter) and is the lightest at 2884 pounds.

But MPG is not the whole story. I wondered which diesel is the most efficient. Without resorting to higher math (partial differential equations came to mind – it was a weak moment!), I’d rather use a “seat of the pants” guesstimate. Pick your choice before you read the next paragraph.


I choose the Dodge truck with the 5.9 liter Cummins. A 4×4, heavy truck with very lousy aerodynamics and the largest engine still gets 24 MPG! What MPG could be achieved if a few changes are made?? Bear with me, these are only interesting guesses.

Configuration MPG

As tested ====================================== 24 mpg

What if I had a 2wd truck? ========================== 26 mpg

Cummins forums routinely have threads about achieved MPG(4). The 2 wheel drive Cummins usually get to 2 to 4 mpg better fuel economy. Conservatively, I’ve added 2 MPG.

What if I had a smaller engine in my 2wd? ============= 30+ mpg

Cummins makes a 3.9 liter diesel used (known as the 4BT) that is used in delivery vans. A website 4btswaps (5) has info on putting these engines in a wide variety of trucks, cars, SUV’s, etc. What if I could put that engine in my truck? The fuel economy would soar (driving easy!) The 3.9, at two thirds the size of the 5.9, would zip fuel. The 4bt would work a harder to propel my 1996 truck, but I’m guesstimated the fuel economy would be above 30 mpg and as much as 36 highway. The 3.9 liter would have plenty of power and torque for what I would tow. (I used the ratio of the engine sizes (5.9/3.9) to guess at fuel economy. Remember, this is just a mental exercise. I’m not towing 12,000 pounds with the 4bt.)

What if I had 2wd, a 4BT and an aerodynamic front end? === 35 mpg

The drag coefficient, written as C(d) , is one factor in the fuel economy of vehicles. Sleek modern cars have a coefficient around 0.30 Lower values are better. I couldn’t find an exact value for the 1996 Dodge truck, but I would guess around 0.60 – a real ugly value from a fuel economy point of view (2).

Reducing the drag coefficient improves fuel economy. “For a full-size truck, a change in drag coefficient of 0.01 is approximately equal to an improvement in fuel economy of 0.1 mpg on the combined city/highway driving cycle,” says GM’s Schenkel.”(3)

I would never make the front end of my truck aerodynamic – it would look like some ugly bullet thingy. But assume someone else wanted to make their truck aerodynamic. Changing the drag coefficient from 0.6 to 0.3 would add (according to Schenkel’s statement) another 3 mpg on the city / highway cycle. Since drag is most important at highway speeds, perhaps 5 mpg would be gained in highway driving..


The 1996 Dodge Cummings is big heavy 4×4 diesel that gets 24 mpg. To compare it to the other diesels in my table, I made some assumptions. I switched it to 2wd (like the VW, MB, Sprinter) and I switched to a smaller engine, 3.9 liter, that is still larger than any of the vehicles in my table. Finally, I made it aerodynamic like the VW Beetle and the Sprinter.

Is 35 MPG achievable? Maybe. Likely. But to find out, someone needs to build it!!


1 From auto.howstuffworks.com

2 From en.wikipedia.org/automobile_drag_coefficient

3 From http://www.edmunds.com/fuel-economy/improving-aerodynamics-to-boost-fuel-economy.html

4 Cummins web sites

Diesel Truck Resource www.dieseltruckresource.com

Turbo Diesel Registry www.turbodieselregisty.com

Dodge Cummins Forum www.dodgecummins.com

5 www.4btswaps.com

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