Currently I have bald 285/75/16 BFG MTs, but just bought a set of 295/75/16 Nitto Terra Grapplers off the internet.

 

You can only recalibrate the speedometer electronically. Your local dealership can do it up to a 285 using the DRBIII scan tool.

With 285's and 3.55 gears you are doing much better than 13mpg, but until you get calibrated you won't be getting the right data to determine your actual economy.

 

I got a 99 shorty 4x4 with 265's, mostly stock and 210k miles on her. She's gettin 18-19 all day long. Tire pressure will effect your milage, my tires say 80psi cold, that's what they are at, rides a little stiff, but I can't complain with 18.4 mpg. I run a Sea foam in the fuel to keep the injecters clean.

 

I have 40 psi in my tires and get 18.5, and don't run sea foam in mine, just 2 cycle oil. I have not noticed any difference when I had 35 psi or 55 psi in my tires.
-P

 

Holy cow you run 80psi in your tires!!!!! What kinda tread wear do you have on your tires? Im going to do the chalk test with my new tires and run em at what ever PSI gives me perfect tread wear.

 

Me too. I run 70 lbs in the front and 80 in the back just like the factory sticker says inside the driver door column. I always use the factory recommended tire pressure rather than what the tire guys want to put in it. Tire wear has been absolutely even and at about 50K on the Michelins. And, I get 19-20 with 4:10s on the highway, 16-17 around town and 13-14 towing a 6-8K horse trailer. Lot more hills up here than in San Antonio too. But, I get better mileage with a fresh oil change and better mileage adding PS to the tank with ULSD.

 

What is PS? Is that a fuel additive?

 

PS - Power Service, diesel fuel additive, Wal-Mart sells it. Can be winter with anti-gel, or summer, just a lubricant supposedly. A lot of discussion is going on here that the lubricity for PS is the same as fuel's so how can that help lubrication, and people lean towards Stanadyne. Had it been more available I'd use it but I don't see it sold retail around here.
-P

 

Yup, just the brand name you can find at Autozone, VIP, Walmart, etc. How many miles do you have on the truck? There are a lot of things that can affect mileage that are out of your control; air temperature, altitude, terrain, etc. There are lots of things on a vehicle that can reduce mileage that may not be apparent - a dry or failing U-Joint or bearing, drum brake self adjusters that are no longer working, even out of balance tires. It all adds up. Try to control what you can. Every used vehicle that I have bought, I change the oil, tranny fluid and filter, differential oil, fuel filter, etc. - even after I look at the prior maintenance records. I've bought certified used cars that had 3K miles on the last oil change. If you are over 100k miles, have the valves adjusted as a previous poster said. If you've changed from the normal tire size, learn how to compute mileage accurately. There are plenty of threads here that will tell you how to do it.

But remember, no two identical vehicles will ever be guaranteed to get the same mileage. These are all production vehicles and assembly line tolerances have a range that is fairly big, which is why you see a range on the new car mileage estimates at the dealerships and why some engines last a lot longer than others given the same maintenance. It isn't only how you drive, but how finely balanced the engine is and that varies widely from block to block at least for gassers. That's why the biggest single performance gain for a pure stock vehicle is usually blueprinting and balancing of the engine - at least years ago in the drag racing circuit. I'd hope Cummins would be built to finer tolerances (and it should be for the diesel compression ratio if for no other reason), but my guess is it's still a wide gap.

 

i have an 01 thats getting 17.5 no matter how you drive it . check your air filter for sure and make sure it is breathing freely. no matter how much i bomb it the mileage either stays same or improves. notice on my signature the gears im running

 

Just bought a 99 2500 qc auto and I got 16 something coming home. I recently got under 14.5 driving to work and back. I am disappointed with that. Hopefully i can figure out whats wrong with it. My tired 318 got 14 all day everyday city/hwy driving with me getting into it all the time.

 

Wrangler04, my tires are LOAD range E 10 plies not range D's, 80 psi cold is stamped right on the side of the tire, you should never run pressure under what the manufacture says.. running the proper tire pressure GIVES you an even wear pattern, any more or less it won't. Load range D's... I'm betting is at least 55psi.

 

I just recently got my truck, 02 2500 Ext Cab Long Box 4x4 with 3.55s and my first tank around town I was averaging 12.5, now on the second tank I'm up to 15.5... which has got me really excited when you consider I came from an 02 1500 with the 4.7 and I drove like a bat outta hell... I was averaging 9-10.5 in town with that...

 

Ha! Remember that while you might not be able to get a lot better mileage than you did with your old truck most upgrades to the new truck give you the potential for better mileage and power and when you are finished you could tow your old truck, beat it in a race with another one just like it on the back and still get reasonable mileage!!!

 

Oh yeah, thats just another reason why I love the CTD.