The thing about Dodge and Jeep Gas tanks i have noticed is Half a tank means you have only around 8Gallons of gas or so. Full to half is about 14 gallons assuming dodge tanks are 23g like my Jeep

If you put $20 in from empty then you werent on the half way mark to start off with. Unless you put $20 in a little under a qaurter that would put you at half. However that would mean you only had around say 8-9 gallons in the tank. But again i dont know what were at when you filled up.

Best way to get your MPG is to fill the tank up all the way and take a note what the mileage was when you filled it up, drive around till your at like half a tank or near empty. Then fill it back up again completely and take the miles you drove and divide that by how much gas you refilled with. This is the only way to get the MPG. The Computer is pretty accurate usually around 90% accurate for me, but this way you can get a clear reading.

8-10mpg seems a little low tho. However many things come into play here. Is it cold where you live ? With the weather being in the 20-30s in Virginia my MPG went from around 13mpg to 11.5mpg. Cold weather really hurts gas mileage. I see you live in Chicago.. what are the temps there ? Like 10f or something right ? Thats why you gas mileage sucks, you have a lift big tires and its very cold.. So i dont think anything is wrong with your truck.

Also im not sure what the Dodge 5.9's require but my Jeep requires 93 octane. Your only hurting your engine by using 87 if it needs 93. It will actually make ur MPG worse too. I suggest spending the extra $3 it costs to put 93 in.

23 gallons of 93 is only like $4 more vs using 87 for a 23 gal tank

 

I got about 9 mpgs around town, could maybe hit 12 on trips with my '98 RCSB 1500 5.9 4x4 for the 4 years and 68k miles I had it. Lifted on 35" tires though...

 

The death flash was meant for other issues, not to cover the plenum gasket leak as previously thought (look at the FAQ, lots of info on the plenum and death flash there).

What gears do you have? Most have 3.55s unless you managed to get a 2000-2001 Off Road Edition. 3.55s are great for MPGs with stock 245 tires, but anything larger and they hurt your gas mileage. 33s are roughly equivalent to 285s. You also have a 4x4 and extended cab, both of which add weight.

I have an electric fan, CAI, Fastman TB, high flow cat, Flowmaster muffler, and I fixed the plenum gasket. That all helps. What hurts me is 3.55s still, and a 93 performance tune off my SCT programmer. I need to cut that back to 91. I am currently getting 9.5 to 10.5 MPGs in the winter, 99% stop and go (less than 35 MPH) driving in the city. I also drive the truck pretty hard.

I just accepted this fact that when I bought a truck, I was going to get bad MPGs. Granted, I was hoping to stay in double digits, but I am not going to hassle my truck over two MPGs. It gets me where I need to go, can go through the snow we get up here with plenty of power, and thats all I ask.

 

Yes I have the 2001 SPORT OFF ROAD EDITION, does that mean 4.10 gears? I drove it on the highway today and doing about 65-70, i was getting 12-13mpg, good or bad?

 

look in your glove box...there should be a sticker there with your gear ratio on it...

 

12-13mpg is not too bad for a 5.9 4x4.

 

That sounds completely normal.

 

MPG in during the winter months is just like the birds...they both go south for the winter. With the combination of the winter blended fuels and the PCM reading the colder outside temps and adding fuel to compensate, mpg will not be back until spring. Just the nature of the beast.