Pskaar, my thoughts on your mileage question. Without knowing for sure what truck you have, i. e. 6 spd. , auto, 2wd or 4 wd, 3. 55 or 4. 10, this is just a general statement. If the truck is still new with few miles (under 10 or 15k) the engine is still not broken in. Also, this is winter where I'm at so we are stuck with a less powerful and efficient diesel fuel, i. e. lower mileage. Also, the specifics of your truck will influence the mileage. You don't really sound that far off of normal. I will average 11 to 14 towing 10k plus, depending on speed, wind, hills etc. Hope this helps a little.