Sounds like a heavy foot problem to me, j/k. With the information you gave, I have to agree with what most everyone else said, sounds about right with the gas mileage. But, I will say 15 MPGs sounds a lot better than 13

 

I'm guessing you have an auto. We never see as good a numbers as the stick-shifters. We just don't get the power to the ground as efficiently. 14.5... empty? Sound to me like you've got a torque converter that needs a little help. Or your brakes are rubbing. Extra tires will decrease mpg's somewhat, but it shouldn't be a huge difference, esp. if you're running skinny's. You gotta remember, out trucks weight an extra 1000-odd lb's than the 250's also. Mine, with a half tank of fuel is around 6200.

 

I think I checked it here bout two months ago and yup your right its an auto. The thing runs out as awesome as can be nice solid shifts and all the gears seem to work good. Like I say had the pump turned up just a little but I think it can still go more. As far as tires I upgraded from the 215/70/16 that were on it when I got it to nice all terrain so no wild tread 235/70/16. I bought a cummins hopin that I would see some of that awesome mileage I had always heard about but I won't scoff at 12 mpg's loaded with 4 ton or more that beats the hell outta my 98 gas stick shift which would get 8 on a good day with the same load. Would any mild budgeted mods help improve the mileage?

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I forgot to mention I may be off on the mileage as a cop let me know last week that my speedo is 14 mph slow! Good thing the stacks took his mind off of the ticket and he just warned me.

 

i had this problem.... stock on all dodge trucks is 245 tires. i'm running 33's on my trucks. thats four tire sizes larger or 8%. i have to add the .08 or 8% to what my trip or odometer reads to get an accurate reading. if anything your fuel mileage should go up... when they geared these trucks your road RPMS are 1800 with a lock up TC. the bigger the tire the lower your RPMS getting your engine to its power band or lugging power. a Cummins aint no V8, it doesnt like running high RPMS. PS your speedo will be off also. with my tire diffrences from stock at 60mph on the speedo i am really running 65-66 mph. the Nice police man with his radar has warned me many of times....

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Tire size calculator just found this... this will tell you the percentage difference in your stock and current tire, and the speedometer correction. it's good stuff. only problem is when you get in the big tires you cant see the percentage.

 

YEAH MY 87 F-250 WAS GETTING 20 MPG WITH STOCK TIRES..... THEN I LEVELED THE FRONT END AND PUT ON 315/75/r16 10PLY TIRES....HANKOOK RT03'S TO BE SPECIFIC....
I HAVE A 7.3L IDI NON-TURBO AND NOW IM GETTING WITH CORRECTED SPEEDO AND ALL 16.5 IN THE CITY AND 20.5 ON THE FREEWAY ALL DAY ..... I HAVE BW..CLOSE RATIO 4 SPD AND ITS 4X4 AS WELL....CUMMINS SHOULD SPANK 20 ON THE HIGHWAY FEEDING 2 LESS CYLINDERS AND WHEN YOU PLACE A HIGHER LOAD ON THE ENGINE WITH A DIESEL YOU SHOULD GET BETTER FUEL ECONOMY..... UNDER MOST CONDITIONS UNLESS YOUR A REAL MANIAC ON THE LOUD PEDAL

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The reason you should be getting better fuel economy is relitivly simple...... when you place a higher load on a diesel it slows the rate of burn down because the engine has to push more to move each piston in order to fire...... creating a higher temp in the cylinder its basic law of how diesel's create power..... but there is a line that can be crossed just like with any engine too much can have the oppisite affect.... i went from 31's to 35's and lost nothing and i drive the hell out of mine and dont shift till at least 3k on the tach........ and im running royal purple 15/40 full synthetic motor oil...... maybe its could also be that i only run shell diesel wich contains 5 % bio-fuel wich burns hotter naturally...look into different fuels and go through a speedo check to get appropriate "actual" milage..... mine was every mile i lost .1 miles

 

In stock form i get 18 empty (6500 lbs) and 15 hauling 5000 pounds in the bed on steep mountain roads and its a automatic, its really better than that since i have 33" tires with no speedo correction. i will say i never put my foot in it unless it is needed.

 

Mine holds 32, the last 2 in the neck.
Did you correct your speedo for the larger tire size? Quite a difference between a 31" tall tire and a 35".

 

does anyone have a non lock up torque converter? if so what kinda mileage do you get?

 

Mine is very much non lock up and I get 18 plus on 33's

 

In the first gens they all had non lock up torque converters but mine gets around 17 mpg.