Baffling Fuel Mileage

[hunter82i]

Afternoon guys. This thread is not to start another fuel mileage suggestions/tips/discussion. Most of us have probably read all of that.

I do, however, have a very peculiar situation that I wanted to bring up. I have an 03 6.0; EGR deleted, otherwise stock. I bought the truck with 54k on it and for the first 50-60k got usually between 17-19 MPG. Needless to say, I was happy with that coming from a Tacoma that got the same. I even had one road trip through OH on Labor Day where I held it around 65 and truthfully got 20.8 MPG. That's 480 miles on about 23 gallons.

Fast forward to a few months ago. I got new tires and decided to go w/ 235/75/16 because next to 265's they were just a little shorter but narrower and I thought that might help out even more. Shortly thereafter, my FICM crapped out and got replaced with new. That is basically where I am now. The difference; I'm lucky to make to 400 miles on 23/24 gallons. Last fill up it was 360.

At $4+ that hurts as I'm sure you all understand. As I said, I'm pretty aware of all the tips/tricks so we don't need to go there if you don't want to. Proper tire pressure, controlled right foot, good maintenance, including fuel filters regularly.

Changing the tires and the FICM in close proximity gave me two variables that are making this hard to diagnose. Does anyone have any other thoughts on why such a decrease? I'm now at 130k miles. Should I just be happy I'm still getting 15+ MPG and move on?

It's just very perplexing how I could get repeated, reliable 420+ mile tanks and now can't get close to that.

PS: one other oddity. In the past, if I filled up when the fuel light came on, it would take about 22 or so gallons to top it off. Now, since the new FICM, I can pump upwards of 24 gallons before she gets full. Most of the time, the $100 limit kicks in. Does the FICM have something to do with fuel level readings?

I've driven the truck every which way over the past few months and the above data is true and accurate without embellishments.

Any ideas? Would something stupid like a stuck thermostat effect mileage or efficiency that much?

[poorboy1964]

did you have your FICM changed at the dealership? if so they might have re-flashed to a newer program version and others have said it dropped there fuel mileage.

[hunter82i]

Originally Posted by poorboy1964

did you have your FICM changed at the dealership? if so they might have re-flashed to a newer program version and others have said it dropped there fuel mileage.

It was not at the dealer but the latest program was flashed. I haven't heard where others have experienced this but kinda hope your right at this point since I can't figure it out

[Armycat10]

As for your FCIM I would go to www.fcimrepair.com and have it reprogrammed. Also if you had the updated flash will cause you to get less MPG. I would be happy with 15+ but sending your FCIM reprogrammed by Ed will give you better gas mileage and better turbo response.