Engine/Transmission (1998.5 - 2002) How does the overhead display compute MPG?

[rweis]

I always thought the input to MPG was the ring gear tone ring and the fuel tank gauge inputs, nope.



I put a aux tank in the bed that feeds the OEM and keeps it full until the aux tank is empty. The aux tank is half full and therefore keeping the OEM tank full. The OEM fuel gauge has not moved in days, BUT the overhead is still computing mpg.



Where is it getting the fuel used from?, the VP44? I have a RASP so it is not the OEM lp either. Has to be from the VP44 I think, where else could it be from?



Ideas?



Part of the reason I am asking is the MPG at cruise (2k) went up significantly. I know the overhead mpg is NOT accurate (mine is about 20% too high, DD2's and Smarty I think), but relatively it increased about 4 mpg (30 mpg to 34 - 35 mpg), now why?



I know that losing MPG is a sign of losing a VP44 as it gets more and more away from calibration and is going toward failure. Many threads about getting back mpg when replaced the VP44.



Bob Weis

 

[2000BlackCloud]

Curious also... I would like to actually try to calibrate mine if at all possible. Anyone got any ideas?

 

[Gary - K7GLD]

I believe the DC display setup uses info from various sensors and the ECM/PCM fueling maps to "guestimate" the expected volume of fuel injected into the engine at various combinations of road speed vs throttle position.



Unfortunately, all that programmed-in info is based upon purely stock fueling and fuel system components - so when bigger injectors and other similar mods are made, there is a corresponding reduction in applied accelerator pedal position vs road speed - and the computer THINKS the truck is moving faster and farther on a smaller volume of fuel than it actually is, thus the erroneous readouts...



My trip to the Oregon coast last weekend displayed 35. 1 MPG - yeah, I *WISH*! :-laf:-laf

 

[rweis]

Sounds like a job for Marco / Smarty for program "0" to change the ECM parameters to calculate the MPG so we can get it closer to correct, sort of like setting tire size.



Gary, yeah, same kind of thing, 35mpg, I wish.



Bob Weis

 

[Bob Cochran]

I have noticed at times my overhead MPG matches calculated or close anyway but only when I have run less than a total of 35 gallons size of OEM tank. However, it I have used say 80 gallons because of TransferFlow my MPG in overhead display is a lot higher than calculated. NOt sure this has any bearing on this topic but might so I threw it out there. Perhaps one of the constants used in making the calculations is 35 gallons?

 

[BTowler]

Not sure on this, gas engines use vacuum to calculate mileage, but no vacuum on a diesel. All I know is I have never seen one that was accurate. I had a gasser 1500 rental when my truck was in the body shop, said 18-22 on the idiot box, hand calculated it came to 14!



I too would love to see 35 out of my truck!!!!

 

[Gary - K7GLD]

The best vehicle mileage computer I have seen was the one in my '85 Corvette - with or without power mods, it was dead-on accurate every time I checked it by hand calculation. But that was a simpler system that involved FAR less sensors and computer programming than our trucks - it just needed the road speed/distance, and how many injector impulses per mile - and with a known volume of fuel per impulse, MPG calculation was easy - and accurate.



The more sensor variables you toss into the mix, the harder it is to provide accurate computation that covers all potential and rapidly changing variables, especially as power mods severely alter fueling and timing. You might get it pretty close in one limited window of operation - and then be WAY off scale when conditions and engine load change.



Providing accurate MPG display for our trucks s not something likely to be easily provided across the wide landscape of varying power mods - Marco or otherwise...

 

[2000BlackCloud]

Logic would tell me that it is always possible to fool it with a smarty etc. If you hand calculate 18mpg... keep changing tire size until it reads correct on the display! May take a couple of tanks to dial it in but as long as you don't change your set up it should repeat. Sounds to easy huh. . lol... . I don't have a smarty to try but if it worked would certainly buy one. . not just for the calibration! but I could justify it in my own head and wifes... HA!

 

[DMannon]

If you programmed in a different tire size than your actual tire size (to get the MPG computer to be closer to accurate), wouldn't that make your speedo and odo inaccurate?

 

[Gary - K7GLD]

Logic would tell me that it is always possible to fool it with a smarty etc. If you hand calculate 18mpg... keep changing tire size until it reads correct on the display! May take a couple of tanks to dial it in but as long as you don't change your set up it should repeat. Sounds to easy huh. . lol... .

HMmmmm - lessee - that means in my case, I'd need to calibrate my speedometer/odometer to a LOWER speed/distance traveled - yeah about 40 MPH when I'm really doing 60 - sure sounds like a great plan to me! :-laf:-laf



I see that "lol" in your post, but you better put a few "smilies" -laf:-laf) in your post - some readers MIGHT think you are serious!