Help diagnose why truck is getting such poor MPG!!

Jmoore1225 · Feb 15, 2012

As topic says, 2008 F250 CC/SB 35's w/ 3.73's... 4inch straight pipe from DP back. Running an SCT w/ the RCD 585. Factory air box w/ a K&N dropped in w/ the lid off. I'm getting around 12.5mpg that's daily driving w/ @ least half the miles @ a cruise between 60-65mph. I cut out idling it @ night before I leave work, helped a little... But where are these 14-16mpg's around town everyone claims they see?
This is babying it, purposely trying keep boost low to get these #'s people claim & I'm NO where near finding them!! Lie-o-meter is claiming mid 19's how is it SO far off?

By the way I'm asking this as more concern as if maybe there is somethin wrong w/ my truck? Then fuel savings but that would be a much added bonus!!!

Jeff@Spartan · Feb 15, 2012

You have 35" tires. Don't expect good fuel mileage.

TripleE44 · Feb 15, 2012

I get about 16 on 37s. Some trucks are just different. A while back on another site there was a guy with a stock crew cab that got worse mileage after he deleted.

Maybe you could try out someone other than RCDs tunes? I'd wonder if tuners have different 'styles' that may yield better MPG on different trucks.

Jeff@Spartan · Feb 15, 2012

TripleE44 said:
I get about 16 on 37s. Some trucks are just different. A while back on another site there was a guy with a stock crew cab that got worse mileage after he deleted.

Maybe you could try out someone other than RCDs tunes? I'd wonder if tuners have different 'styles' that may yield better MPG on different trucks.

16 around town????

forcefed6.4ford · Feb 15, 2012

Pay more attention to your rail pressure. 9-9500spi is where I get my best mileage. What is your gear ratio?

MINK · Feb 15, 2012

My lie o meter says 19-20 but its fos....Mine idles alot in winter so I dont even watch the mpg anymore - i'm also on 35s, I would guess im around 14 just of quick math.

TripleE44 · Feb 15, 2012

SPARTAN said:
16 around town????

Just my normal driving. Probably about 60% hwy, 40% city. I didn't calculate this on a long road trip, with a wind to my back, all downhill like a lot of guys seem to. I drive conservatively, but not with mileage in mind. And I really doubt my truck being a supercab can make that much of a difference.

BFT · Feb 15, 2012

There is nothing wrong with your truck. You have 3.73's with 35's on a truck that probably weighs about 8k, and to top it off you probably are still running winter fuel

Jeff@Spartan · Feb 15, 2012

TripleE44 said:
Just my normal driving. Probably about 60% hwy, 40% city. I didn't calculate this on a long road trip, with a wind to my back, all downhill like a lot of guys seem to. I drive conservatively, but not with mileage in mind. And I really doubt my truck being a supercab can make that much of a difference.

That's pretty good man. Most people with 37's don't see that kind of fuel mileage mixed driving.

Jmoore1225 · Feb 15, 2012

I'd be estatic w/ 14 around town!!! Thanx for the rail pressure tip .... I'm not looking for great, just what everyone else is claiming. Ive been wondering if it has somethin to do w/ the RCD 585, as Ive read that it is fuel heavy, that & the pedal is very sensitive! Its all I've got to run right now besides the canned tunes tho!

Forcefed what kinda mileage do you see towing? Come spring & summer I'll be towing my boat, it & the trailer weigh in around 11-12k lbs... Praying I can @ least make it all the way to Lake Cumberland w/ out stoppin to fuel up, about 250miles.... Old toW rig was a 98 DRW dodge ram w/ the V10, she couldn't quite make it.... Averaged around 7mpg!!!

Jmoore1225 · Feb 15, 2012

Thanx for the peace of mind everyone....

BFT · Feb 15, 2012

Id try to keep your RPM's down rather then FRP (fuel rail pressure). Being that you have a big tune you will most likely see 10-12k on your FRP.
Your in city mileage will suck due to larger tires and gearing. Not having a heavy foot and trying to keep you RPM's below 2k will help especially on the highway

SEABEE08FX4 · Feb 15, 2012

Mine always gets the best MPG at 2000 RPM's , never looked to see what rail pressure is at that speed though. I have the 30.5 gal tank though and average 560-600 per tank, I fill up once a little over a week. Although I will say I gained some mileage after the turbo swap but I was still doing better than what your seeing.

I will say though as said earlier some trucks just don't react the same as others, and I'm not sure what your total mileage is but I'm at 174k so its nice and broke in too. If you have relatively low mileage it will get better over time. Also make sure you don't have a clogged up air filter and or fuel filters, those two can affect mileage quite a bit.

Jmoore1225 · Feb 15, 2012

I'm @ 96k.... Usually do pretty good @ keeping her under 2000rpm, I'm lucky to get much over 350 miles a tank. I washed the air filter bout a month ago, maybe I just need to replace it? Changed the top fuel filter about 2k miles ago... Didnt do the filter on the frame as I just got the truck & didn't have a clue where it went @ the time of changing. Will get on that this weekend, also doing EGR full delete.