Fuel Economy - Ford Diesel


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Fuel Economy with a Ford Diesel - Every Bit of Fuel Savings Adds Up!

The picture below shows a Ford E350 diesel powered van.  Just to give you an idea on the type of fuel economy on this Ford diesel rig, it was purchased used out of state and driven home cross country and fuel usage carefully measured.  On the return trip at mostly highway speeds, the fuel economy of the Ford diesel 7.3L Power Stroke engine was around 18 mpg.  Later, this van was modified to run dually rear tires.  After this change, the highway fuel economy dropped to around 16 mpg.  It appears that the extra rolling resistance of the additional 2 tires might have lowered the fuel economy of this Ford diesel.  In addition, the aerodynamics of the van (whatever little aerodynamics it had) might have been reduced further by adding the duallies, since the extra tires stick out the sides of the van. 

Fuel Economy Ford Diesel

Ford 7.3 Power Stroke Diesel Dually Van

The 7.3L Power Stroke was a great engine, but as is often the case, progress moves things forward and results in changes.  The venerable 7.3 was phased out in the 2003 model year, and the Power Stroke 6.0 engine took over.  Unfortunately, the 6.0 engine was never as good as the 7.3.  After a few years, the 6.0L was replaced by the Power Stroke 6.4L turbo diesel engine.  More recently, Ford came out with it's own diesel engine design, and this Ford 6.7L engine is supposed to have some improvements over it's predecessors.  One of the areas of improvement is supposed to be in terms of fuel efficiency.  In terms of the Ford E-350 van fuel economy, another factor could have been that the size of tires selected for the dually conversion had a slightly smaller overall diameter than the stock tires.  All this combined with the very low 4.10 rear axle gears made for less than optimal fuel economy.  However, one winter a set of snow tires were put on back of this van.  Not dually, but just a single set of snow tires.  In this case, the snow tire size chosen was bigger than the original stock tires.  As a result, the diameter of the tires and the overall gearing of the van was increased over stock.  This higher gearing combined with the single tires out back really helped improve fuel economy.  Instead of getting 16-17 mpg while highway driving, the fuel economy of this Ford diesel jumped up to 20 mpg.  Pretty good considering the size of this van.  That was the first time this particular Ford diesel van cracked the 20 mpg fuel economy level.

Free Flowing Exhaust

High Flow 4" Exhaust System

A bigger exhaust system was also installed on this van.  The Diamond Eye exhaust was a full 4" all the way from the turbo to the exhaust tip.  Since this van did not come with any catalytic converter, particulate filter, or other emissions related exhaust equipment, it was relatively easy to make the exhaust change.  The bigger exhaust did improve fuel economy around 1-2 mpg overall; however, the additional noise (especially at highway speeds) was very annoying.  Eventually, the stock exhaust was put back on in favor of a little quieter driving conditions.  The diesel engine is still far from quiet, but at least the resonating drone was gone when switching back to the original exhaust.  It has been said that the noise is not as much of an issue on a truck since the cab is up front and the exhaust runs under the bed and exits out back far behind where the passengers sit.  On the other hand, with the van, the exhaust runs the full length of the passenger compartment and it seems to transfer more of its noise to the interior.

In addition to the exhaust, a tuner module was added that gave some different tuning options.  There was a setting for towing, some settings for performance gains (+ 100 HP!), and also a fuel economy tune.  This economy setting was supposed to change the fuel injection timing to help improve combustion efficiency.  In addition, this economy tune setting also changed the shift pattern of the automatic transmission.  Basically, it shifted up sooner and was slower to downshift to keep the tranny in a higher gear more often.  This helped keep the engine RPM's lower which in turn should lead to better fuel economy.  In theory, this is all true.  In reality, there was no big difference in fuel economy while using the economy tuner setting.  Perhaps it was because even in the economy setting the tuner added more power, and it was hard to drive it without tapping into that extra power more often.  In other words, with the extra power on tap, it was easy (more fun) to accelerate harder rather than driving more conservatively.  In the end, the best fuel economy for this Ford diesel van was right around 20 mpg (driving like a grandpa).  Usually, it was closer to 18 mpg for highway driving.  A big part of this limit is probably the 4.10 gears.  With an axle with taller 3.55 gears, it would probably be possible to see better fuel economy on a Ford diesel.  Then again, with the higher gear ratio, you will also be limited with the amount of weight that you can safely tow.  There's always a tradeoff.

UPDATE:  Well I finally bit the bullet and made the change to the 3.55 ring and pinion gears in the rear axle.  Actually, I ended up paying a reputable shop to to do the installation for me.  Being a motorhead that has some experience with wrenching on cars, I still did not want to tackle shimming and setting up a rear axle because the consequences are too great if things are not done precisely right.  If the gear mesh is not set up just right, then the gears will destroy themselves in short order and you can end up with a larger repair bill.  If that was the case, then so much for saving money with better fuel economy if the repair bills eat up any savings that would have been seen.  In any case, I did my research, shopped around for the best deal, and bought genuine Dana parts.  In addition to the ring and pinion gears, bearings, and shims, I also bought a Dana posi-trac unit.  I decided to have a limited slip unit installed at the same time, since everything would already be taken apart for the ring and pinion change.  I don't know what Ford was thinking, but most of the E350 vans come with open differentials which means that it's easy to get stuck in low traction conditions.  In past, I've had times that I couldn't even make it up my own driveway in the winter!  PATHETIC!  I've had to get the tractor with a trucker's chain and pull the van up our driveway when conditions were slick.  Now with the posi-traction rear axle driving both sets of rear wheels, traction is greatly improved and the van just trudges through snow.  With that being said, switching from the 4.10 gears to the 3.55 gears, I gained around 20% in terms of fuel economy.  On a route over a mountain pass where I'd usually get around 15 mpg with the duallies and 4.10 axle gears, I then got closer to 18 mpg with the 3.55 gears.  Granted, a 3 mpg increase may not sound like much, but when you consider it as a percent increase, then you see that a 20% increase in fuel economy is nothing to sneeze at.  Again, this is with a 2000 Ford E350 van with the 7.3L Powerstroke diesel engine. 

One interesting point of trivia on my particular Ford Superduty E350 van, the rear axle housing is a heavy duty Dana 70 but the internal gears are from the smaller Dana 60 axle.  When I spoke with someone that has been in the axle business for 40 years, he said there was no way that could be true.  He said that it had to just be a Dana 60 axle.  Well, then a miracle occurred because it is true.  It's not very common, but on some of the Superduty E350 15 passenger vans, Ford did specify a full floater Dana 70 axle housing with the smaller 9.75" Dana 60 gears inside.  To me, it's a bit nutty to do it this way.  If you are already going to put a full floater Dana 70 axles in there, why not just leave the standard 10.5" Dana 70 gears in there.  My best guess is that some Ford engineer figured they could save some money by going with this "hybrid" Dana 60/70 axle set up.  Besides the fact that I'd have to keep arguing with the differential shop guy about the fact that Dana 70 gears and posi would fit right into my particular axle, I decided to just stick with the Dana 60 gears and posi because I would have had to pay almost DOUBLE to get the larger Dana 70 parts and stronger posi unit.  If I was doing a lot of heavy towing and had lots of cash burning a hole in my pocket, then I'd prefer to have the stronger 70 series parts in there.  For my purposes, it was not necessary and I had better uses for the money.  One final note on the axles, there are many E350 and E250 vans that come from the factory with a true Dana 60 axle.  There are Dana 60 full floater and semi-floater axles that were available from the factory, BUT on a select few E350 Superduty 15 passenger vans, they specified the hybrid Dana 60/70 axle.  Best I can figure, the Dana 70 housing was chosen for the increased structural strength and load carrying capacity.  To cut costs and save money, they just specified the Dana 60 gears inside because that was all that was needed in terms of strength of the drivetrain.  Structurally (weight carrying capacity) they wanted the additional strength of the Dana 70 axle housing.  By the way, I did verify that it was a Dana 70 housing in more ways than one, but one clear giveaway was in measuring the larger diameter axle tubes.  Well, enough talk about Dana axles housings!  At this point, most of you are wondering what in the world this has to do with anything.  I found it interesting and worth sharing for other motorheads that might also find this interesting.  It's not very often that you see such an arrangement on a factory supplied axle.              

Back to the topic of the fuel economy of the newer Ford diesel trucks.  For those of you interested in the fuel economy of some of the newer Ford F-350 diesel trucks, then you might be interested in the below showing how the Ford 6.7L diesel engine compares to the Dodge and Chevy diesel trucks in terms of fuel economy.  This 6.7L diesel engine was designed by Ford and puts an end to the longstanding use of the International designed and built Power Stroke engine used in the older Ford trucks.  The fuel economy of the Ford 6.7L diesel engine looks to be impressive.