My 1998 12 valve has only small mods done to it, like 4" full exhaust no cat, fuel plate from BD #3 (I believe) and k&n stock replacement air filter. With an auto tranny it used to get 18 or 19 mpg hauling my race car on a flat car hauler on the hwy. I just recently took a trip hauling my harley in a small inclosed trailer and my average mpg was about 12 or 13 mph. The truck runs with good power and doesn't show exhaust except under heavy throttle. Egts and engine temp were normal and boost seemed normal with the altitude changes. The car trailer by far out ways the small trailer. Has any body any idea why this sudden change in mileage. I tried changing to a paper air filter but no change. I thought about maybe a valve adjustment being needed, other than that I have no ideas. Any others suffer this same problem? Any comments are welcome.

 

How big of an enclosed trailer? A lot of people don't realize how much wind these trailers will catch and how much more fuel the truck will use hauling one. What size tires are you running on your truck?

 

The trailer is 7' 4" in height and 6' 2" wide and weight is unknown but must be 1200 at least. There is a slight rearward angle to the rear half way up in front. I have hauled this trailer before and got better mileage but at the time I had a fiberglass shell on it. Air sounds reasonable. I just think it would make such a difference. At 75 mph I guess it could?

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I forgot my tire size is 265/75 R 16 Perrili STR

 

I averaged ~15.6mpg at 80mph with about 400lbs in the box in my duramax not long ago. It was all due to headwind the entire 200mile trip. Even empty if it's really windy that can happen very easily. I'd advise trying another trip and seeing what the result is. If it happens again I'd check for codes or leaks.

 

Without the shell on the truck milage will go down towin a cargo trailer big or small. They catch a lot of wind and when you're goin 75 remember that a 5 mph wind is hitting the nose of the truck and the trailer with 15 times the force of the wind

 

This was a 1000 mile trip to oklahoma (from sierra vista, az) and there was some heavy wind in certain areas. Especially I-25 and I-40. I think I may be going back. Sounds like the reliability is there just the barn door effect is killing mileage. However my friend has a 1997 12 valve (almost identical to mine) pulling a 26' car hauler and averages 19 mpg with his truck. Same tire, gear, tranny except 5" exhaust and different air filter. Is this just the way it is or something I can not figure out.

 

A lot of people fib about mileage. 19mpg hauling a 26" car trailer? I hardly doubt it unless the trailer is empty and an aluminum flat bed.

Try slowing down some and see what it does. I know my CR hauling a trailer that is 8.5' wide, 8' tall and 24' long only got 10-11 averaging 75 with a lot of wind on the trip. Plus I think if you had atleast 285's or 305's it would help you out some.

 

Yes I realize that, unfortunatly we (being drag racers) have traveled together and used the same company fuel on many trips and his truck has always done better for several years now. It has been brought to my attention that not all the rearend gears are as marked. Meaning not the same ratio from truck to truck. Has anybody else heard of this. I also just looked over the specs for valve adjustment and have reservations about how the last person to do this may have been off the mark. Will this have an adverse effect on mileage or just lacking in power?

 

well for one i would say if your valves are not right you wont be getting the power you should and there fore would not get as good as mileage, but then again if your valves were way to loose i think you would hear the valve train noise.

 

If the valves are set loose it could effect power, but like mentioned, you would hear it too.