I took the 2005 3500 out for its first ride with our 25ft Airstream. Huntsville, AL to Mtn Home, AR via Jonesboro.

While the truck exhibits better milage than my '99 CTD without the trailer (21+), the milage with the trailer seems to be the same or a little lower... even though I'm towing at a slightly lower speed (60 vs 65+) and I've gone from an automatic to a 6 spd. And here's the crazy part... I seem to get better milage when I'm in the hills driving in 5th at 50 mph (16-17mpg) vs driving in 6th on the flat at 60 mph (14-15 mpg).

The only other variable would seem to be fuel.

Am I smoking the wrong brand of cigarettes, or could there be that much difference in diesel fuels??? Or, have any of you experienced the same thing?

Confused Engineer.........
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sounds like you are going off of the overhead dont do it it lies

 

Sorry....... I'm an engineer and I have zero trust of the "overhead". These are actual hand calculated milages. The "overhead" varies so badly it's a wonder that they could peddle them. My '99 overhead acutally was "close" but the 2005 is totally out of sight. In fact I'm working with one of my guys to try and talk with the bunch that fabricated the **** things to see if it can be calibrated.

I have picked up on some pretty wide swings on mpg depending on conditions and possibly fuel. The 2005 seems to be much more tempermental about this sort of thing than the '99. Is this a common fault on the newer setups or do I need to have my computer checked? Engine has 75K on it.

 

I actually trust my overhead. Its within 1/2mpg accuate from my hand calcs.

 

Doesn't resistance (drag) go up as speed increases? Can't remember the formula... the square of something or other... Anyways, Probably a lot more drag on the trailer at 60 mph than at 50 mph on the straight and level. Also while in the hills, on the downhills, I will bet you are coasting or near coasting quite a bit. I have found that letting off the pedal and coasting on downhill stretches increases my hand calculated fuel efficiency.

 

What rear gear? 3.73 or 4.10? If 3.73 your lugging in 6th gear at 60 mph.Sweet spot is nearer 1900 rpms.

 

I would think that 50 in 5th is a more efficient use of engine/tranny/gearing than 60 in 6th, areo-friction withstanding.

I just pulled an empty dual axle enclosed trailer from Denver to Amarillo. All down hill except for the Raton Pass. 65-75 MPH most of the trip (Based on Speed Limits) and I made 13.5 MPG in OD. I don't operate in 3rd because engine RPM stays close to red-line, and Tow/Haul mode prevents tranny hunting in the pass. My MPG also droped precipitously when I did make an attempt at third, but I have taller gearing.

 

Running 60 mpg in 6th gear is not lugging these engines. Peak torque is @ 1600 rpms which is right @ 50 mph in 6th gear.

 

Peak torque may be at 1600 rpm but that is not where they pull best or get the best fuel mileage towing.

 

He's pulling an Airstream. I wouldn't think there would be a whole lot more drag between 50 and 60 pulling one of those. Especially if he get's better mileage at 50 in the hills than at 60 on the flat. Doesn't make sense to me.

I'm jealous of his mileage though. I get 11 in town driving like my Grandpa, very rarely going over 2k rpm. Get around 16 on the highway. I'm only at 8400 miles though, I'm still holding out hope that it will improve, especially as fuel prices are approaching $3.50 per gallon here.

I do get 13 mpg towing my 23 foot Timberline travel trailer with the bed full of wood, bikes, and camping gear. I'm happy for that.

I've said this before on this forum, but I wish you could buy a programmer that could de-tune these engines to get the mileage of the 2003 SO trucks. I think it would be a huge seller. Most of us buy these to get good mileage and tow. I'd gladly crest the grade towing my trailer at 45 mph if I could get 17 or 18 mpg average.

 

I only said it's not lugging the engine to pull @ 50mph/1600 rpms in 6th gear. Unless you're up or down shifting on a steep grade and trying to maintain momentum your rpms should always drop down to 1600 on upshifts. It's called progressive shifting and is how you shift for optimum fuel economy. Once you're in 6th then run up to your sweetspot rpm-wise and maintain that as best you can.

 

I see you fellows have as many questions as I do......

50 in 5th equals about 2000 RPM
60 in 6th equals about 2000 RPM

So you can see why I'm confused. The gentleman is right there is a squared function effecting milage at speeds over 55 (that's why they set the speed limit there years back). Actually it is supposed to hit you about 1 MPG/10 MPH over 55.

Given that I have a cap on the truck (crew cab/short bed) and an Airstream, the effect of aerodynamic drag is minimal between 50-60 MPH (yep, I'm a rocket scientist....)

Given that I got 20 MPG on the highway and 15 MPG with the trailer with my '99 using an automatic you would think milage would improve with the 2005 with a stick. And it does without the trailer, and especially in town... but to loose MPG on the highway really puzzles me. I'm wondering if my computer is screwed up somehow (I know that NEVER happens.)

Any more thoughts are really appreciated, as well as knowing if there is a way to calibrate the overhead......

Ralph the Confused.