LMM: Mileage doesn’t make sense

[Spank]

Im scratching my head trying to figure out why my mileage isn’t where it should be, my dash is telling me 11-12mpg around town which is low IMO but my edge insight is saying 13-15mpg in town, around what my last truck was getting. This truck is just chugging fuel. I want to say it’s my tune to blame or they are starting with the winter diesel. Any thought? I check my fuel rail pressure and my balances rate all is well. Mods down to the truck are in sig.

 

[Chevy1925]

Hand calc it. Neither are accurate specially after tuning

 

[Pure Diesel]

Hand calc like James said. My display is off by 8% with tuning. When I was completely stock the display was right on.

 

[Blackmax123]

13-15 is about what my lml dually got. I think that's just part of owning a dually.

 

[THEFERMANATOR]

Hand calculate it. I've tuned lbz's and had the dic be off by over 20%. It's not like lb7/lly where you could correct the dic fuel readings, lbz+ is all handled in the bcm, and is only as accurate as the info it receives from the ecm which is thrown off with tuning.

 

[Ron Nielson]

Lots of fans of 'hand calculating' mileage. So what is it that the truck doesn't do correctly when tuned? Fuel used? Mileage? Calculation? I've asked this question on several discussions, never a direct answer. I'm sure there really is one.

 

[2004LB7]

The ecm/bcm depending on year calculates fuel used by mm3 of injected fuel. When you play with the pulse width of the injectors on time it changes the actual mm3s of injected fuel.

The computer thinks it is using less or more mm3 of fuel when in actuality it may be the same, more or less depending on the tune. Its like re-defining the definition of a gallon without telling the computer it has changed. It still "thinks" that one mm3 of fuel is one mm3. But may be 1.5 mm3 or 0.75. all depends on how the tables where altered

The LB7 & LLY had tables in them for making corrections to get the fuel used right but with the later generations having this calculation done in the BCM we can't make that correction (Ben may be able to, not sure).

 

[Ron Nielson]

Thanks. Just what I was looking for.

 

[Mikey52]

So is it possible to tune the bcm?

 

[2004LB7]

See here

https://www.duramaxdiesels.com/forum/showthread.php?t=75287