Not a Vortec, but IF I am Lucky I have gotten 12 w/my 93. K3500, NV4500, and 4.56's. Haven't checked lately due to lots of idle time for winter "Warm Ups"
I got about 17 on the highway stock and I got 14 on the highway all the way to BB03 with 39.5's... However I drive like a little old lady so I tend to get better mileage than most other people.
My dad thinks he gets about 12-14 average with his '98 C30 dually equipped with an NV4500. I can't comment much myself becaue I don't drive it much but I know it gets better mileage than my blazer does with a SBC.
My 87-ish 1 ton crew cab dually w/ 7.4 TBI and turbo 400 gets 6mpg towing about 15-18k lbs....48' car trailer w/ 3 vehicles on it....gets about 8-9 mpg empty...and thats with a magnaflow muffin and k&n filter...
I guess I should feel lucky 89v3500 7.4 tbi 3 inch exhaust...intake mods as suggested (removal of the air silencer)..k&n a carefully built motor with lots of time and money invested it gets best tow across country 10k pounds... 8.4 worst 5.7 (killer winds and hills) empty 10 when my right foot is disciplined ....who knows when I experience the thrill of chevy power.... but its significantly less cam
Vortec 454 mileage: My '99 4x4 Dually got 7mpg, then I sold it...I'm guessing it had a flat cam or something (it also lacked power). My '96 K2500 Suburban gets @ 10-11mpg, but would get better if I could keep my foot out of it!! Towing, the Sub gets @ 7-8 in the mountains and 10-11 on flat ground.
