Have a 2006 3500. When it had only 10,000 miles, it would get around 21 - 22 mpg highway. Now under same driving conditions, I feel lucky to get 15 - 15.5. Towing a 14,000 lb 5th it use to get 13, now I'll get 9 - 10. The truck is stock 5.9 with 5 speed auto. I guess it's time to take it to the dealer. Any ideas of whats up with this drop in mileage?

 

Check your air filter, and if you haven't, change your fuel filter. Those are what affects my MPG. Also, I noticed that winter fuel gets a little worse mileage than summer fuel.

 

check the fuel supply and/or check the torque on the connector tubes.

 

1. winter fuel.
2. fuel filter
3. bad injector

 

What part of the country are you from? Something I never see addressed here is the fact that these engines seldom run at high enough engine temp. this causes mpg problems, mainly from low combustion temps. I wish someone would come up with 2 things: 1) An aftercooler bypass. 2) Automatic radiator shutters. I recomend covering the radiator not the front grill. I work mine a fair amount and can keep radiator fully covered til 40F. Even in the summer time my engine rarely gets to 200F. Keep these at a temp beween 200 - 220F you will see better fuel economy. If Auto Tans equiped be sure to keep cooler unblocked.

 

Injector pump would be a good place to start looking. Mine was leaking, so the dealer replaced it under waranty and mpg's went up.

 

I have noticed this winter that my city mpg has stayed the same at about 16 but strangely my highway mpg never goes above 17 unless I drive about 55mph. In recent years I always seemed to lose in the city but not so much on the highway. In the summer months I normally get around 19mpg highway with a best ever of 23 at 60mph sustained...
All my filters are new BTW...

 

I have been using #2 diesel, a new air filter and fuel filter was installed within the last 5,000 miles. The truck now has 35,000 and is driven mainly in warmer weather (in s. Texas now). The lift pump was replaced under warranty at about 20,000 (was leaking). With the price of fuel going up, I want the good old days when I was getting higher mileage.

 

#2 can be winterized w/out labeling.....

New tires? Tire pressure?

Is that mileage hand calc or overhead....

 

Had new Michlens put on about 4,000 miles ago. Was hoping that that would help. I tow alot so I keep the front tires at 60 and rear at 65 psi. The mileage is hand calc. It appears that the overhead is off from 3 to 7 mpg.

 

Just curious. You really have a 4 spd don't you? I'm not an auto guy, but I thought they were all 4 spds unless it was the new 68RE?

 

The 48 RE is a 4 speed...