yeah I would check all the above. My truck is/was the same as yours and I was always getting at least 17mpg combined city/hiway but I'll tell you what is eating your milage the most is spinning the engine @ 2400-2600 on the hiway... with the torque reserve these engines have, you can spin 1700 @ 70 and get much better fuel milage. When I got rid of the sloppy TC, put in a getrag 5sp OD trans, I took it on a trip from L.A. to Canada and cruising at no more than 65 throughout I logged by hand 25 mpg 24 gallons & 600 miles @ fill-up!! my truck had a 3" exhaust and stock air filter setup at that time...
Now with 4" and bhaf I have never seen below 20 mpg with fuel turned way up blowing black smoke everywhere all city , whatever. I am leaving on a trip to L.A. right now. I will let you know how the mileage is on this trip. I like the non I/C trucks, esp the engines! I have a 54 mm compressor on, with a 16cm exhaust housing now as well. I now have the fuel set up with AFC starwheel mia (spring still in) and fuel pin at deepest setting. max boost is around the stock 18-20 until it rises with rpms and 3200 gsk all the way up to 26psi max @ over 2600 rpm... I think the full fuel screw is turned back to stock. I also have about an 1/8" timing bump. KSB is hooked up

 

my light usually comes on at 300 to 325 miles.
it ran out of fuel one time at 394 miles.

 

Best i ever got is 31 empty flat N.D in a 350 2 whd. air cond on high ,winter goes down to 17 - 20

 

Best i ever got is 31 empty flat N.D in a 350 2 whd. air cond on high ,winter goes down to 17 - 20


How did you maintain control on such a long downhill grade in neutral? Seriously, What injectors, what converter and /or transmission, and how many miles on the truck? Thanks.

 

Best I got was 19 on the interstate. Normal is 16 unless Its in 4x4 a lot and that pulls it down some. Worst is 10-12 hauling a load. That all said, I know my mileage is off - I'm going 5mph slower than what the speedo says. Which I think (if I did the cawlkulashions right) means its a little worse than that.

I feel good when I get 400 miles before the light comes on

I attribute my poor economy to the TC and maybe some of the mods.

 

I used to get 24 highway and 17 to 19 pulling a load.Now around 16 hwy and 10 pulling a load.They loss may be the t/c.

 

Most people just think we are crazy to live in the frozen weather.

My milage was up to 13.9 (hand Calc) today. That's the best tank since last Nov. I've been seeing around 9 MPG (no sense doing the math when you know your gonna get less than 10 mpg and your going to fill it up anyways) most of the winter. Winter blend fuel, lots of idle time and 4x4 always locked in is killer on the milage.

 

-45 this morning truck inside not pluged in #1 with 50% kerosine it puts mileage down to nothing but it starts and takes 30 mlies to get any heat.Anyone that dozent plug there truck in is not doing thereselves any good.cold starts are hard on the motor.have 0-20 syn oil in motor, take a can of 10-40 oil in feezer and see how it pours,it turns to greese.#90 axle oil is soild.Try std.oil and syn.in freezer and you will never use std oils again.