Gas mileage algorithm?

[Rich Z]

On my 2002 Corvette Z06, I can view a real time display on the DIC showing instantaneous and average gas mileage while driving down the road. Does anyone know what values the PCM is using to determine those figures? And how accurate is that display when modifications are made to the engine, particularly the fuel injectors? I'm guessing it merely needs to know how much distance was covered in a particular time frame and how much gasoline was used during that time. So how would the 'fuel used' value be determined? Fuel injector duty pulse? If so, what tells the PCM that new injectors are delivering more fuel per cycle than stock when the stock injectors are changed for higher flowing ones?

I'm curious to know if what I am seeing for gas mileage right now is actually "real world".

Yeah, I can figure out average MPG by simply keeping track of mileage on my gasoline receipts, but I'm mostly curious about that instantaneous display.

[joecar]

I have a calc pid for separately calculating instantaneous MPG based on injector flow rate...

copy the attached calc_pids.txt file (which has the MPG pid merged into the Calc.VET pids) to the User Configuration folder...

then start the scantool, goto the PIDs tab, select the pid CALC.ECON.MPH or CALC.ECON.LPHK

( in the PIDs tab, on the pid ECON, do rightclick->More Info to see which other pids require to be selected for logging )

[Rich Z]

Thanks, Joe.

So does this mean the MPG shown on my DIC display is not accurate? Or is it accurate and the calc_pids entry just allows me a way to capture that info?

[Boost]

That's awesome Joe. Rich, I find that one of the things that affects the accuracy is of course extended idling (fuel is being used and zero miles travelled). Sometimes my customers will comment that a truck is getting poor fuel economy, tell them to reset it on the highway and the average settles about 5 MPG higher for weeks. When you idle you can watch it drop in just a few minutes.

Of course this doesn't affect Instantaneous. That would simply depend on the accuracy of all the variables. For instance, check with a GPS or an Speedometer App. on your smartphone that the speedo / odo. is calibrated correctly. I think that would have a huge effect on the readings. Personally I feel that GM is a tad optimistic when it comes to the display.

[joecar]

The DIC and the calc pid depend on the IFR table being correct...

if the IFR table has been scaled then neither will be correct.

[Rich Z]

The DIC and the calc pid depend on the IFR table being correct...

if the IFR table has been scaled then neither will be correct.

My IFR table values match the data given to me by the injector builder. Since I'm using a mapped FPR, the IFR table is a flat horizontal line.

So does that mean the DIC values I see for instant MPG are correct then?

[Rich Z]

That's awesome Joe. Rich, I find that one of the things that affects the accuracy is of course extended idling (fuel is being used and zero miles travelled). Sometimes my customers will comment that a truck is getting poor fuel economy, tell them to reset it on the highway and the average settles about 5 MPG higher for weeks. When you idle you can watch it drop in just a few minutes.

Of course this doesn't affect Instantaneous. That would simply depend on the accuracy of all the variables. For instance, check with a GPS or an Speedometer App. on your smartphone that the speedo / odo. is calibrated correctly. I think that would have a huge effect on the readings. Personally I feel that GM is a tad optimistic when it comes to the display.

Yeah, I hear that. I've been doing a LOT of idling lately trying to get log scans during cold start up so I can try to figure out why the engine is idling so rough while in open loop. Best I can figure out, engine rpm is bouncing around because spark advance is jumping around.

[joecar]

My IFR table values match the data given to me by the injector builder. Since I'm using a mapped FPR, the IFR table is a flat horizontal line.

So does that mean the DIC values I see for instant MPG are correct then?Yes.