Literally from one tank, I lost over 3 mpg, and haven't gained it back in the last 4 tankfuls. I went from around 13.5, and am struggling to maintain 10 at this point. I haven't been driving the truck any different, haven't been pulling anything. I also have an incredibly rough idle. At first, I thought it was the fuel filter, but that is brand new. Also have gone to different gas stations thinking it was bad gas, but no luck so far. Where should I start looking to solve this problem? Thanks in advance.

 

Has it been to the dealer lately? I ask because mine has also droped by about that much.

 

Rough idle sounds like you lost a coil on one of the plugs. I would be very happy to get 13 MPG. In the hills of PA, I get 10. In the flats of MI, 15.

 

I had the same thing happen to me, come to find out that I had a broken gas cap. Go figure?? Give it a check... Judd

 

Some bad gas could have plugged an injector, you could havew a bad coil or even have carbon build up on the valves all could cause your problems. Take it to a shop and have the coils checked and also have them run some injector cleaner, the good stuff not the stuff you pour in the tank, it hooks to the fuel rail then you disconect the fuel pump and it runs off the can. This will clean any build up in the injectors and remove carbon deposits on the valves. You should see a big difference is that was you problem.

 

I only have 7,000 miles on my truck, and have seemed to only get 9 to 10 mpg. It runs great and has a ton of power. Is this normal for a 2004 F- 350 with 430 rear gear ratio. Or should I be concerned about that kind of Mpg.

 

cclgeo while 9-10 is a little low it is not outside the range of reported average MPG.

Tell us more about your truck, where you are, how you load and drive it.

The V10 has a sweet spot rpm that is most effecient, unfortunatly it is just around 2000 rpm in OD with 31 inch tires and 3.73 gears.
Under easy acceleration, and steady long runs in cruis control we typically see 12.5 to 14 MPG in this area. The folks with 4.10 get a little less and the 4.30s take a bigger hit due to spining the motor up around 2300rpm in this scenario.

Folks also report, and my own experiances comfirm that most of the time the MPG difference between 65 MPH and 75 MPH is very dramatic. Going a little faster costs a big hit in MPG.

The other big factors are fuel and tire preasure. Cheap or blended fuels tend to hurt the V10 performance with most of us reporting significant differences between some brands. As an example around here I can get RaceTrack, Citgo, or on base AAFES brand fuel for about 3-5c less than Shell, Texaco, or Chevron. I have proven to myself over many tanks and long range trips that Chevron w/techron yields the best average MPG.

Tire preasue, low will ride easier but hit the fuel effeciency a lot, fully aired up to maximum for the weigth yields a significant increase.

Some folks report a full 2 MPG gain with a tonneau cover, my experiments saw only 0.4 MPG gain.

 

Last time I had a drop in MPG with my V10 it was the fuel, I figured it out when we were out of town and refueled, the milage jumped up 2MPG while towing. Change stations and brands it mite help, I was saving 2c a gal when I had the problem.

Denny

 

Well with the 430 gears the rpm's are much higher at freeway speeds. I live in San Diego CA. and everybody drives 75 to 80 mph. So I'm turning some high rpm's to keep up with traffic. The truck is stock, and has the original tires at the right psi. Also I only use Chevron gas in my truck. The only thing I have done is change the y-pipe out for a more true y-pipe, and install a flowmaster 50 seris 3" muffler. The truck really gets up and goes. But wait I also used the leveling kit on it, so now it has a 4" lift all around. As I'm thinking right now, I think that it felt like I was pushing a brick through the air when the lift was done. Since the nose was not down anymore, it might have changed the aerodynamics of the truck, and change the MPG. But off the top of your head what is the average MPG a 2004 F350 4X4 should get.

 

My 4x4 did not really break in until I had around 13K on her. You should be up around 11 MPG but at those speeds you are never gonna get to 13-15mpg until you get the effective ratio back to around 4:1 with taller tires. This should bring the rpm back to 2000-2150 range where the motor is most effecient.

4 inches higher will cause a small amount of added air resistance but since it already is a big brick I doubt it is the main culprit...Those speeds are killing you MPG.

 

I went from my CC 2X4 with 3:73's and was getting 14.8 - 16 on the hwy to an Excursion 4X4 with 4:30's and am running 13.987 just about every time. Now I figure that only on hwy driving, from Lafayette, LA to New Mexico a little over 1000 miles with no stops between fill ups then back. Then from Lafayette to Glendive, MT 1900 miles and I drove up non stop other than fillups, on the way back I stopped for some sleep. Again I am running between 70 and 77 MPH with the cruise set and not stopping but for gas. So IMO the 4:30's are not killing me on MPG and I am running stock tires.

 

Well I have an update. Took a 225 mile trip to go fourwheeling over the weekend. Going there I got about 11 mpg. Filled up there at a Conoco with 89 blended with 10% ethanol, as all 89 octane is around here. Made the trip back, and got 13.8 on the same stretch. Did some driving around during the week, and filled up at a Shell station, the same one I used since I bought the truck. Got 89 octane again, and that tank was 13.4. BTW, I have used 89 octane blended with 10% ethanol since I bought the truck, what would have caused this decline, even after switching brands? And why did it go back up?

 

Stringer, I don't know how to explain it, my truck does similar things. I have a 16' utility trailer hooked up to mine 4 to 5 days a week and it varies from 7.2 to about 9mpg with no difference in driving, almost all in-town driving. The only explanation is the gas guage is more like an hour meter The more distance I cover in a given time, the better the mileage. I am convinced if it sat idling for four hours it would run out of fuel Recently I hauled a load of fencing and shingles down to a buddy in Orlando. I had a pretty clear run for about 40 miles, the rest of the tank was my usual in-town type driving. Mileage spiked from 7.7 the previous tank to just shy of 10mpg. Hard to believe that little bit could make that much difference, but it seemed to.

On the subject of brands of gas, mine certainly seems to be easier on some than others. I have heard Chevron offered several people here a little advantage. Not so in my case. Chevron has consistently been one of the worst for me. These are not Chevron corporate-type stores, but rather independent convenience stores. Maybe that has a lot to do with it. The best my truck ever did unloaded was on old cheap FlyingJ 87 octane, almost 14mpg on the trip back from NY. In my day to day normal driving, Hess, so far has been the best for me.

 

I get 10 on hwy 8 maybe city 13 or 15 would be awsome you guys must live in flatland

 

Surprisingly mine did better on fuel in more "hilly" areas on the way back from NY than it does here in Florida. Go figure