I have 05 5.9 2500 4x4 with about 140,000 on it now! When the truck was bone stock the first year I drove it, it did 17-19 hwy, and 14-16 city. Never any better, and only worse when towing or added weight in the back! A 600lb wheeler, or a 1,000lb trailer load would drop it 15-17 and 13-15 mpg! Before I purchased this truck every diesel owner I knew said that added tow, or weight did not effect mpg, but that was not he case, but still not bad for a 325hp 610 foot pounds of torque 8,000 lb truck! After all were not driving a Prius here.
After 15,000 miles I added 35in tires a leveling kit with 2 in block in the back and super chips flashpaque tuner, for a Lil more ooommff! Set up like this the truck does 14-18 hwy 12-14 city the higher mileage naturally is empty truck with less fuel, and the lower would be with a load on the truck! Since the day I bought the truck the computer lied about the mileage, along with my close friends 05 5.9 and my uncles 06 5.9. Mine stock said 21-22 mpg hwy, and 17-19cty my calculations did not add up to this ever! With the programmer it said 23-25 hwy 19-21cty, again the computers lying! I've always calculated my mileage as long as I can remember, and then out of curiosity on fishing and boating trips doing the same on my buddy's and uncles. Then again their's was also not accurate! I've been in 6 different 5.9's 4x4's from 04-06 models and none of them did better than 20 hwy, but all respectfully close to my mileage some being stock, 2 being 6 speed manuals 4.11 gears! The 6 speed manuals did the closeest to 20 hwy empty! The 4 speed autos a Lil less with 3.73 rear ends. Either way you look at it, if you do buy a 5.9 your getting a great motor in a decent truck! 2 of the trucks I'm very familiar with are pushing 400,000 original miles on the motors! Tranny for those years are another story! That Lil motor is tranny destroyn machine! It pulls like a freegin freight train!
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