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I know without a doubt that my 99 got better mileage than my 07. The only difference is the 99 was a SC and the 07 is a CC.
I could easily see high 13's pulling my boat with the 99. Same trip with the 07 I have yet to break 12.
Love the extra power and the 5 speed though.
And the 99s I was around typically came off the showroom with 3.73 gearing, big jump from 3.73s to 4.30s. I wonder how well a 3v would do with 3.73s? Regardless, I wouldn't trade the extra mileage for the loss in towing performance. I pulled a 27' V-Nose enclosed trailer up Hatchers Pass today and averaged 8.1 mpg. The Duramax definetly had at least a couple mpg advantage while towing.