Last week I went on a trip to Colorado to ride ATVs. I towed 2 machines on a trailer just big enough for 2 and a third in the box of the truck. It was a perfectly flat drive til we got to the mountains. All I could manage was 12 mpg at 72 mph. I wanted to drive abought 80 but I couldent stand to see the fuel milage drop any more. When we got there I parked the truck and it sat for 3 or 4 days withought geting driven. Thats the longest its sat sence I bought it this spring. When I went to start it it ran real rough and not on all cylinders for a second then smoothed out and smoked (white smoke) for a long long time. The temp was down in the 40s or so. I had problems with the low temp dead spots in the throtle back when it was cold. I had the reflash done and sence then it has not been cold enough to tell if it workd. When we where runing around out there it droped to 37 and the dead spot returned wors than before. I am thinking a few injectors may be bad. It goes in friday to be checked out. When I was talking to the service manager he did not seem to think 12 mpg was bad. I had to practicly force him to check it out. I was hopeing someone here could put there 2cents worth in and tell me that a $40000 truck should get better milage than this pulling no more than 3000#.

On a side note I thought I was getting away lucky with no driveline vibration but at one point I hit 80 for a few seconds and heard and felt this weird vibration sound. It took a few seconds to set in that it was my truck. I had dismised this problem because it had never hapend at 70-75 like it was suposed to. Does anyone else have the noise at higher speeds like mine? Has anyone figured out the cause?

Thanks for the help. You guys are far more knolegable than my dealer at this point. They just seem to learn stuff as I tell them, and then they tend to look a little baffeled.

 

As far as the milage, when I had 7K on the clock I towed a 6000lbs TT, up a 6% grade (4-5 miles)WOT up the hill and averaged 11MPG as well as about 70MPH

I passed everything but a filling station...

Regarding the vibe, I have it bad!!!! For me the faster I go the worse it gets at about 85 I feel like the truck could blow at any time, I don't go faster than that!!!
Tom

 

I have 6500 miles on mine now. I have not noticed any vibes until now. I don't know if its a vibe or some kind of noise (harmonics) in the exaust. It seems to do it around 2300-2500 rpms. not speed related. It seems to be worse when the a/c compressor kicks on. I am waiting on a cat-back system from DPP , if this does not help , back to the dealer I go. Some have said their vibes went away when they changed the exaust . I hope this right in my case. I think that there are several different vibe problems out there.

 

I forgot to say that I pulled 3 ATVs on a tandem axle through the hills of Tennessee at 70-80 and averaged 14mpg. This was at 2500 miles right after I had all 6 injectors changed for engine miss since new.

 

I'm curious to know if this turns out to be bad injectors... I think you may be on the right track there. I just read another thread about that... https://www.dieseltruckresource.com/...threadid=47111

It won't be cold enough here till like Nov-Dec to see if I get the white smoke, etc. but my truck does exhibit a weird knock and gets really lousy mileage. Last time I towed my boat, which is 5K# fully loaded including the trailer, I only got 9MPG @ 72MPH cruise going mostly flat or downhill, with very little climbing. I can also say that the performance while towing is less than stellar with this truck.

As far as the vibe goes... I had it bad when the truck was new... but ever since I removed the exhaust mount to the tranny and installed the 315's it's been much more civilized. It's not gone completely, it just seems tamer... either that or it's now too high in the speed range for me to notice unless I'm in a really big hurry! I'd still like it fixed tho... I'm gonna eventually try a new exhaust and if that doesn't work maybe try these... http://www.balancemasters.com/driveshaft.html

 

Is there anyone with the vibration who gets good mpg's? Is there any connection to the vibration and low mileage? just asking I don't have the vibration and I get good mileage.

 

bkrukow, if it makes you feel any better, I towed my 3200# boat on flat land last weekend for a few hundred miles and only got 11.1 mpg. I get 10.4 mpg unloaded in city driving. I've been trying to be light on the gas pedal, but still no improvement. Granted I only have 1900 miles on it, but have not heard of anyone getting this bad mileage even with this low mileage.

 

I went to Oklahoma City last week to pick up my Elite 6-horse and going down with the cruise set on 75mph I could get 14mpg. Coming back with my trailer with the cruise set on 75 I got about 10.5mpg. Driving around empty around home if I drive like a grandpa I can get 17mpg at about 55-60 mph. My wife can squeeze no more than 14.5 out of it empty. I find these figures to be better than what my 01-03 Chevy Duramaxes got doing the same jobs. I sometimes got as low as 7.5mpg pulling my 6-horse trailer. I am happy with the 10.5 running with the cruise set on 75 pulling a 6-horse trailer. It pulled every hill from OKC to back home on I-44 in overdrive, never once kicked down.

 

Aerodynamic drag has a lot to do with it. That drag also increases exponentially when you increase the speed. FWIW

 

The last trailer I pulled was a small (2) horse trailer and @ 70- 75 MPH I got around 14.3. The interesting thing, I though was a few weeks before that I pulled my jeep on a car hauling trailer together weight was around 5 1/2 to 6K at 70 MPH from Dallas to Corpus Christi (400 miles). I averaged 15 MPG on the trip down and 13 on the way back. Coming back we were driving through a thunder storm . I seems to me that as soon as you put any type of load on these trucks the fuel mileage declines rapidly. Empty around town I get 18-19 and hwy 20-22 if I stay under 2,000 RPM .

 

Tonight on the way home from work I took the senic rout and went threw town. I could hear a knock type of sound that its had from day one and Ive been thinking its getting worse. After reading the other post I will ask them to check into that as well. Hopefully they can figure it out.

As far as wind drag, I reolize that it more times than not is what hurts fuel milage and not the weight of the trailer. The two fourwheelers where on a low trailer down behind the truck and did not create a hole lot of wind drag. Now the camper I pulled to wisconsin and back a month ago was a lite trailer but it created a lot of wind drag and my fuel milage sufered. I only got around 9 mpg, but I was runing between75 and 80 all the way.

 

You guys got those new enviro-friendly trucks, but my "old" '03 HO gets 13.5 pulling my boat - 60/40 highway/city. GCW is around 16.5k, but the boat is aerodynamic.

I don't like pulling faster than 65-70 but will if I have to pass someone. Once I get her up to speed I'll do my best to ensure I don't have to slow down or stop (its that follow-on acceleration that kills mileage).

 

Pulled 11800# TT to Mich and back to VA. Able to get 12.4 or so on way up. Pushed harder and faster coming home and was able to let it see the high side of 11..... about 11.8. Speed normally in the 65 mph range with some above. Auto trans. Was able to get the automatic to bark a couple of times being lazy. Have to watch that.

 

Well I got it back from the dealer tonight and they claim nothing is wrong. They said they took it for a ride and got 17-18. I drove home after that and drove as slow as I could stand (58 mph) and kept it in the 17s. I dont know what to think. Part of me says that it should have gotten better milage than 12 at 72mph pulling 2500 pounds, and the other part of me says maybey its just the nature of the beast. Its like its got a switch on the hitch that atomaticly drops the milage to 10-12 no mater what is behind it.

On a side note to add insult to injury, the truck I traded for this one is back in with bad millage. Its a 98 24 valve auto 4x4. He recently drove it 150 miles and burnt 30 gallons of fuel. I felt awfull, That was a great running truck when I traded it. They dont have a pop off tester for the injectors there and they think one stuck open. They said it barely ran.

 

737 miles pulling a 27' Arctic Fox at 75 - 80 mph when not in traffic. 04.5, six speed, 3.73, 4000 miles. 7.01 mpg. I have changed the fuel filter and still no change.

Even with some improvement I am going to require a much larger fuel tank.

Maybe I'll take my 12 valve out of my 1990 V3500 crew cab and swap it into my new truck. Maybe I shouldn't sell the Cummins Chevy.