First long trip - mileage questions...


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I took the 2005 3500 long and tall on it's first long trip since the "conversion", and was kind of surprised with some things.

My interstate gas mileage truly sucked. I was probably in the 70mph range most of the time, and really never got better than 18mpg or so. Normally, I get around 22mpg on average....

The weird thing, was up in the mountains of CO, I was usually getting more like my old 22mpg, with all the mountain climbs, stop and go through town, despite often traveling at the same 70mph speeds.

Seemed like the opposite of what I would have expected. I don't know how much being at the higher altitudes, or maybe even a different fuel blend might have made a difference.

I really didn't have the van all that loaded. Maybe 500lbs have been added. I did have the AC going quite a bit on the interstate though.

Does all that seem right? I ask, because I'm still a little concerned with leaks after replacing the resonator. I also had some AC work right before I left, but I really hope that didn't change anything...


New member
I recently got 25mpg+ driving around in CO, back roads from Aspen to Leadville. I suspect that you may be driving slower on average than you think. A GPS can keep track of your average speed, reset the trip calculator when you fill up and compare it to the gallons used. I also had a nearly one ton load while getting this MPG, and the return trip to CA empty averaged about 22, which is normal for me; I was also going 70-75 most of the way back, and 55-60 going TO CO.


New member
I'm assuming your 2005 was actually a diesel, even though you use the term 'gas mileage'. A/C uses a fair amount of energy. The summer it was hitting 95-105F every day, I got lower mileage compared to cooler summers. I suspect at higher altitudes you were using the A/C much less. Also, at 70 mph, air resistance dominates the equation and thinner air makes a difference.

With gas vehicles, highway mileage tends to be higher than around town. Not so for diesel because of their efficient idle. My mileage drops 15% at steady 70 compared to around town.
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I've taken a couple of long trips in my 3500, including two that took me thru Colorado. Mileage averaged 20-24...typically the lower numbers corresponded to higher speeds on the freeways. I get my best mileage between 55-60 mph. Driving around Michigan I can pretty much depend upon getting 23-24 mpg.

My trips were during times of the year where the AC was seldom required. That said, my mileage was often less using winter blend diesel. I tend to drive very smoothly with gentle acceleration and plenty of time allowed for stops. I monitor engine load with my Scangage II and manually downshift anytime load starts to push 90%.



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I have a 2005 class B camper on a 2500 chassis with just a little over 30K on the clock. On a recent trip to Wyoming and Montana, we averaged 22-25 mpg. The camper was loaded close to the GVW limit with water in the tanks and all the rv stuff like generator, fridge, microwave and all the personal stuff my wife and two young daughters could fit in their duffels. Also, I seem to think that the #2 diesel gave better mileage IMHO. On the day we took the Bear Tooth Hwy Pass (Almost 11K Feet) from Yellowstone and into Billings, Montana, that tank gave us an average of 25.5 mpg per my Scangauge II. The best on that trip.



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In the summer of 2007 I had one tank of bad diesel fuel that knocked down my mpg a lot. It took two or three tankfuls to get back to normal mpg. In 2008 no problems. However, this summer on a 5600 mile trip I have had three tanks of bad diesel. I wonder if bad fuel could be your problem too.

Dick from Florida

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