International 9900 58"or 72" mid roof

The engine trans and gearing combination is such a big factor that it's almost impossible to make any kind of realistic projection of mileage. Any time you have a large hood truck, with air cleaners out in theairflow and the exhaust up in front in the airflow, then you will get worse mileage than you would if you had a more aero truck.

To get the best you can out of it, get the exhaust out of the airflow and back behind the cab. The 9900 uses a smaller diameter air cleaner than many other similar trucks, so there is some help there. If you are going to pull a van, get the wheelbase and fifth wheel position set so that you have no greater than 30 inches between the cab and the trailer front, 24 inches is better and below that if you can make it work. If you can't achieve that, then use cab extenders to close down that gap. Pulling a van, you need a full height air deflector on top of the truck. Set up your gearing so you can use LP 22.5 tires on the truck, which will reduce your rolling resistance as much as possible. Put synthetic lubes in the rear ends and the transmission to reduce friction. MAke sure your trans is a direct drive rather than an overdrive unit. Combine all these things with a reasonable selection of the engine and trans combination, and you'll get the best mileage possible out of the truck it you drive it with mileage in mind. Limit idling, shift progressively, use every fuel saving driving tip you can, and you'll get as well with it as can be done.

Whether or not it will hit the 6.9 mpg or greater range is hard to say without more info. Hope this helps.

 

In an OD trans, there is an extra shaft turning inside the transmission that rotates more gears through the trans fluid than a direct drive trans. In a direct drive, essentially, the gears are all taken out of the equation and all you have spinning is the bearing at the front and rear of the trans supporting the main input shaft. So no gears are turning in a direct drive system. It's estimated that you lose approx 1-2% in your fuel mileage running an OD vice a direct drive. So technically, if you are doing everything possible to gain fuiel mileage a direct is the way to go.

In real life, I would never let it be an issue for such a minor fuel gain. Buy your used tractor and don't worry whether it is a 2.93 and a direct drive or a 3.73 and an overdrive.

But, the point of the discussion was how to get the best fuel mileage out of a "classic" styled truck, and while the difference is minor, it is there.

 

My dad drove one for a while with a 72" mid-roof loved it had a detroit 500 with straight easton 10 speed he averaged between 5.8-6.4mpg all depending on conditions.

 

The problem with direct drive transmissions is that engine makers are requiring cruise RPM's that are hard to achieve with direct drive without using rather tall rear end gearing.

 

Basically, I agree with you. And if I were buying a truck, even with the goal of maximizing fuel mileage, I would probably disregard my own advice, because for the prospect of a 1-2% fuel savings, I think I would rather have the luxury of a wider selection of vehicles to choose from, whether direct or overdrive trans.

But the point was that we were asked a mileage question without an engine, trans, or rear end choice, which made answering it very general at best. So I reverted to the basics of getting decent fuel mileage, which is all that;s really left on the table. mechanically, there are less shafts turning in high gear in a direct trans, small though the difference is. And that;s why I suggested what I did. Had we had an engine choice and the rest of the driveline, the advice may have been quite different.

 

According to a Mercer Transportation news letter, regardless of the type of truck you drive, studies show that you can reduce fuel consumption by lowering your speed. The reason is that up to 60 m.p.h. there is a “pillow of air” that builds around your truck and continues to flow over the truck. When speed exceeds 60 m.p.h. the air resistance or “pillow of air” must be split and displaced, which progressively demands more horsepower as your speed increases. Any time you double a given speed, your wind resistance multiplies 4 times. By reducing speed from 70 m.p.h. to between 60 and 63 m.p.h., you can reduce operating cost and fuel consumption by one mile per gallon. Over a period of one year, this results in a substantial savings in fuel cost.

 

True enough. What might be bad for a CAT C15 ACERT will be just fine for a Cummins ISM. Too many variables out there nowadays.

12, 14 and 16 speed transmissions with direct drive are very popular in Europe but they run at 80-90km/h (50-56 mph) due to strict governor rules and tachographs.

Plus they're going to want to run as efficient as possible since they are paying $5 or more per gallon. When a full tank of fuel is going to cost you near a grand, you might be willing to run 50 mph too.