I am looking for suggestions from the group on any ideas for better fuel economy out of my 99 3/4 ton 4x4 5.9 gas pick up. I know that a 3/4 ton 5.9 is going to drink some gas and I am not expecting 18-20 mpg on the highway but there has got to be a way to squeeze a couple more mpg out of my truck!!! Here is the lowdown: 3/4 ton 360 gas, stock motor, 4.10 gears, 4x4, 106k miles, Only item changed is a K&N filter system I installed. Stock tires and wheels and nothing else.....Bone stock. My best mileage to date has been 13.2, combo of highway and city, since the K&N I have also replaced the plugs and now I get 11.5!!!! And that was all on the interstate!!!! Going from NC to Tenn I only got 11.5 and I double checked it to the overhead console and it was the same. I am seriously considering the dyna track hub switch kit to do away with the full time hubs so I can get the lock ins lock outs but its expensive and moneys tight right now. I am not expecting a whole lot out of my truck but if I could manage a couple more mpg that would really help, any suggestions are greatly appreciated.................Thanks

 

Before spending any money on things that don't work as far as MPG claims. Do you know if the plenum gasket was ever checked or replaced? And when was the last time it had a good old fashioned tune up? And the fluids replaced in the diffs ,TC and trans?

 

And BTW, forget checking your MPG with the OH console, do it the old fashioned way.And depending on where you live, the cold parts of the country has additives in the gas in the winter, and the MPG will suffer.

 

I have not changed any fluids except the oil, diffs and trans have not been changed yet, tune up was just new plugs the wires looked ok and I also did not change the dist cap or button. I will do all that as soon as the snow melts that we got hit with last night. I also was wondering if the 4.10s are hurting the mpg also? I have heard 3.55s are better for some mpg. Thanks for the info...................... By the way how do you know if the plenium gaskets are bad?? It runs great...........

 

Here's how to check the plenum gasket, http://dodgeram.info/tsb/2000/09-05-00.htm

And as far as the gears go , it all depends on what type of driving you do mostly, as in highway or city. I do mostly city and would kill for 4:10's LOL. But that's not your problem if you were getting better MPG before.

 

What size tires are you running?

When I had 285's and 4.10's I was getting the best MPG's I have ever gotten (16 mpg).

 

You're getting about the same as I'm getting with my 1/2 ton, 5.9L with 3.55's. And, just like you, mine was better before I replaced the plugs, wires, cap, rotor, and coil. My plenum was leaky, so I changed the gasket. It's not leaky now but the mileage did not improve.

In my case I'm starting to suspect the wires-- I got zapped when brushing up against the coil wire while checking something w/ the engine running. These are the cheap Advance Auto wires...

Try replacing your old wires with good quality new ones.

 

Thanks for the response, I will look into the wires and cap and plenium gasket, I am also going to swap to synthetic oil for the diffs maybe that will help?? I also plan on opening up the exhaust system some with a cat back low restriction muffler and get rid of that bathtub muffler on it now................thanks again.

 

i wouldnt bother changing gears,,truth be told youd prob hurt your mileage worse changing to 3:55's from 4:10's,,as it takes more power(more gas lol) too keep your speed up..going from 3:55's to my 4:56's im actually get better gas mileage now,,as i barely have to touch the pedal going out the hghwy..