I'm trying to figure out something. My scan guage and my on board computer vary up to 3 mpg. Are they the info from the same source? Anyway I know the most reliable info comes from pencil and paper.
However I'm not sure why I should go from 16-18 mpg solo ang 8-10 mpg towing to 11-12 solo and 6-7 towing. An
M I missing something?
2004.5. 6.0.

 

You might have picked up some bad fuel, especially since this happened out of nowhere.

You may also have your first tank of winterized diesel. Winterized or #1 diesel has less BTU and kills fuel economy. Consider stopping by Autozone and picking up a bottle of Stanadyne Performance Formula (blue label). It costs $7.99 and will bring the general quality of the fuel up.

You may also consider picking up a scangauge to monitor your FICM voltages as this could also be a reason why your fuel economy is dropping. If your FICM voltages are less than 47-49 volts, the power side of the FICM must be replaced.

 

Forgot to add, I ha ve scanguage and everything checks out.

 

When you are talking fuel econemy ther are so many variables that aren't even mechanical it is hard to say what all may be causeing it.

 

I know. The mileage I have noticed is over a period of time. The figures used are on level terrain with not much wind. In fact the other day I was towing with wind to the back. The on board computer said 10.9. The scan guage said 7.8. I didn't have time to stop and fill up then do the arithmetic. I was mostly wondering why those two would vary so much. I assume they both get readings from the FICM. Just curious and wondering if I should be watching something I don't know about. Also I was doing around 60 mph and rpms about 1975. I did some miles on tow mode and some in overdrive. Didn't notice any diff

 

Even the quality of the fuel and useing cruise or not can come into play.
I recently rented a car to go to a funeral. Going we used a high grade fuel negligable wind either direction. No cruise couldn't find it LOL
Comeing back we filled up at a walmart 10% ethanol and figured out how to use cruise. Used 3 gals more gas going than I did comeing back. Was a gasser tho.