I recently had a 6000 km journey to Florida, all interstate driving at generally 5 over the posted speed limit.
Six tanks of fuel were used.
Each fill up had an impressive DTE which I assume is based on mileage I was getting toward the bottom half of the tank.
The actual miles traveled to the next fill up was always considerably less (1350 km/1000 km for example)
It seems to me that the KAM is not following along after shutting off to get fuel, each new tank starts high then works it's way down to calculate the next DTE.
Normal? Bad PCM?

 

Its not a KAM issue. Your DTE calculator is using a formula based on a factory preset value for the fuel consumption rate. They are all too optimistic from the factory. There are several threads discussing correcting the DTE.

 

For 2021 they have reworded the DTE to ODTE

 

so using DTE wasn't a good example. After shutting the engine down, should ones mpg be significantly worse than it was 15 minutes prior?

 

There are several different types of MPG you can monitor. Which one are you referring to?

 

the running fuel economy setting, i usually run on it seeking which gear to be in, which rpm is best for the speed i'm going. All goes out the window whenever you stop

 

After running your engine for a period of time, the fuel system goes into a closed-loop control using feedback from the O2 sensors upstream of the catalytic converters. When you shut the engine off, the O2 sensors cool off. Restarting the engine, say after 15 min, the fuel system runs in an open-loop mode until the O2 sensors get hot enough, then the system reverts back to closed-loop. Closed-loop, in general with a healthy fuel system, is more efficient and will show a better fuel economy that open-loop operation.

 

the dealer checked the O2 sensors but is it possible the O2's are working but faulty?

 

very doubtful.

 

replaced the battery, seems to run more efficiently