this is what I know. We bought a 2001 3500 8.1 dually with a 4:10. At that time the duramax was also the first year. the test was going up a test climb to compare both engines. The 8.1 was quicker. the difference was the duramax was a 3.73 rear gear and pulled the test climb in one gear higher in the Allison. Now we have the 05 same model as the 01 truck and it is a superior tow truck. the 8.1 was not bad for fuel for towing and 7.9 was the lowest mileage but in a windy day towing from Palm springs though El central to Yuma. 13.52 was the only mileage I ever checked solo on a 234 drive one day filling up to the brim at the same pump both times. 9.30 mpg was the best on one perfect day on I-5. I have talked to other 8.1 owners and they didn't get up to 7mpg towing. I always did the best test I could do because no reason to lie to myself, in fact I do not lie period. I would pull the hills around 3500 rpm's with 4000 a couple of times to see the difference but not my style to drive that way. I go with what feels good to me is good for the truck. The 8.1 was great for us but we decided we had too much money so the reason for the 05.
chevman
