I have an '05 2500 4x4. I bought it new and now has about 132,000 miles on it. I have always thought that typically, with higher mileage, the better the fuel mileage gets. Not so with mine!

I commute about 100 miles a day (highway) and when I tow anything, it is my bass boat (just about every weekend).

The first few years, I was getting about 18-19 (21 best) on the highway and 15-16 towing my boat. Now, I am getting 15-16 on the highway and about 12-13 towing my boat. What is going on?

 

I will add that the only modification is an Air Raid kit.

 

I've notice my mileage has dropped also but not as much as yours. I use to get 20 mpg easy (combo of city/highway) but now 19 mpg. I think it's the fuel. I now buy diesel with about 10 to 11 % bio mixed in (b10) were as before it was straight diesel or b5. I've been told bio diesel has less energy per gallon. I'm no expert on this topic.....

 

I've read a lot recently about how ULSD has impacted a lot of truck's mileage in a negative manner. Some are seeing between 15-30% decrease in their mileage.

I'm not sure if this is the case in your situation, but it could explain it if the drop happened around 2006/2007 when they introduced ULSD.

 

That timeframe would make sense. I added a BullyDog Triple Dog and have been running it in the Tow mode. I hope at the end of the week I will see a nice increase in mileage.

 

I'm still undecided of my BDTD gave me any extra economy...

My problem is I like the extra power I've gotten out of it, so of course drive it harder... which offsets the gains in efficiency...

I did however get good economy towing when I kept my speed around 60mph..

 

Well, one week of highway driving (to and from work) and then one weekend road trip with the Bully Dog, the results are up from low 16's to mid 17's. That is not what I was expecting, but is better than I was getting and the extra power (when needed) is incredible.

I checked my codes with the Bully Dog and there are none.

 

x3 on the ulsd, hasn't been the same since. when in Oregon i talked to some cummins owners who had been using the state mandated diesel #1 which is 20% biodiesel, and they claim 3-5 mpg lost on average. I would hate to see the impact on the 6.7L's