Fuel Economy Misconceptions

[MichaelRubin]

I Hear Many People Quoting Fuel Economies That Sound, Impossible Unless They Are Towing Downhill With A Tail Wind The Whole Time.



Any Truck With A Electronic Trip Computer That Calculates Fuel Economy And Also Has A Performance Chip, The Displayed Fuel Economy Will Be Incorrect.



The Computer In The Truck Calculates The Fuel Economy (mpg) By Tracking Fuel Consumption In The Following Manner:



The Pcm Sets The Fuel Injection Pulse Width And Pressure Based On A "look-up Table" That Takes Into Account Rpm, Throttle Position And Several Other Variables. The Pressure, Timing And Pulse Width Are All Hard Wired Into The "look-up Table" In The Pcm.



The Fuel Economy (displayed On The Overhead Console) Is Calcualted From The Distance Traveled And The Volume Of Fuel That Is Sprayed Into The Cylinders As Per The Specifications Hard Wired Into The Pcm Look-up Table. The Computer Knows The Exact Volume That Gets Sprayed Each Power Stroke.



Now Here Is Where The Problem Comes From:



The Performance Chips Intercept The Serial Data Stream From The Pcm That Tells The Fpcm How Much Fuel To Inject (and When For Timing) And Modifys The Pulse Width And Timing Data With Settings From An Alternate Look-up Table Contained In The Aftermarket Module. The Truck Pcm And Truck Trip Computer With Average And Instant Milage Is Still Calculating Your "assumed" Fuel Economy When The Truck Is Reallly Spraying More Fuel Into The Engine That The Stock Pcm Called For. This Is Why A Lot Of People Think Their Milage Is Higher Than Really Is.



Fuel Milage Needs To Be Hand Calculated If You Have A Performance Chip. Your Stock Pcm Is Being Fooled Into Thinking That You Are Running With Different Fuel Timing And Different Injector Pulse Width.



This Informaiton Is Directly From A Design Engineer At Edge Performance.



If Anyone Has Any Questions Or Comments, Please Post Them And I Will Answer.



BY THE WAY, I HAVE A 2005 2500 DUALLY 6 SPEED ST QUAD CAB NOW, THE INFO BELOW SHOWS MY OLD TRUCK

 

[JVolpe]

I knew pretty close on how the overhead calculates from the start. I just wish mine was closer to the actual mileage. I have hand calculated mpg on every tank since new(28,000+ now). Stock the overhead was about . 7 mpg low. After the TSB 18-037-04 the overhead was about . 9 mpg high still stock. Various performance boxes both pressure and timing/duration have altered the overhead variance greatly, all higher than actual hand calculation. Now with the Juice/Attitude and EZ stack the overhead is about 2 - 3 mpg high depending on settings. At least I have seen over 20 mpg readings on the ovehead lately, just wish it was closer to actual.

 

[DHayden]

That info has been known basically since trip computers and mods came out. I always do hand calculations and suprisingly between my truck being changed over from being a Canadian truck and all my mods. . the over head is usually only about . 4 miles per gallon off.



That's pretty lucky for me but obviously others computers are way out of the ball park. Hand calculation is the only way to know for sure.

 

[67guzzler]

This is slightly OT but without mods my overhead read 16. 4 on the last tankful and I actually got at measured 16. 24. Pretty darn accurate.

 

[Matt400]

I Hear Many People Quoting Fuel Economies That Sound, Impossible Unless They Are Towing Downhill With A Tail Wind The Whole Time.

True but also many don't realize how much the driver plays a part. My 03 got 19. 5 hand calculated and now that my son owns it and he gets 16+



Many have complained the newer ones get bad mpg cuz they have so much more power and I am finding my 05 is getting 18+ and its no where near broke in.

 

[Blakers]

The van Aaken C3. 2 box does not throw the overhead off.

 

[Tim]

My truck isn't even modded and the overhead is wrong all the time.



Please don't capitalize the first letter of every word. It's really hard to read.

 

[MichaelRubin]

I was typing on my laptop while my friend was driving my truck in the middle of the night on rt 80 in pa, I am lucky I was able to type at all, but point well taken.

 

[gsbrockman]

My truck isn't even modded and the overhead is wrong all the time.

My truck's overhead was usually within . 2 or. 3 mpg of hand-calc's prior to the addition of the EZ (now it can vary as much as 2. 0--2. 2 mpg). Do you have Spooled-Up's Fuel Tank Vent Mod ? If not, some of the variances could be due to inconsistent re-fueling of the tank every fill-up. I've noticed a slight difference in foam at two or three places I typically buy my fuel at.

Greg

 

[Nightram]

I just took my fuel economy the handwriten way, and I got 19. 6 mpg at 2,800 miles. My computer said 21. 3 mpg, so it is a little off.

 

[JTreas]

my overhead says 14. 5, almost through its first full tank of gas. thinkin about throwing some sand bags in the bed to help put some weight on it to help with the break in, think that would help it?

 

[nps]

My OH computer was accurate until the EZ.



With the 315 tires and no spedo recalc, the OH computer is accurate again +/- . 2mpg.

 

[lmills]

never pay that close attention to it other than to judge how hard I have been running it.

 

[ELK311]

I have a stock everything and the console always reads high when compared to manual. food for thought

 

[DIESELMAN]

I Hear Many People Quoting Fuel Economies That...

Flame proofing engaged.....



Now that post reminds me of my wife 'getting' a joke I told last week... .

 

[DPKetchum]

The times I check mine and I reset every tank its withen . 5 or closer.

 

[JPereira]

I use the above console as a guage. It gives me a quick view of how the truck is running. If it usually reads 18. 5 and when I look at it and it shows 17. 0 I know I am using more fuel than usual. When I want to be accurate I use my calculater and odometer. My mileage whenever I state it is from my calculater not the overhead. Every tank gets written down and calculated in a notebook. One thing I never really hear much about is coasting to save fuel. This is the first diesel I have owned and was amazed how far you can coast without touching the long pedal. I have been driving to work for about a year and still try to see where I can let off and coast to end of street, off ramps etc. I think this is a large part of my saving fuel almost as important as keeping it under 70 on the highway.



Jeff

 

[Tim]

My truck's overhead was usually within . 2 or. 3 mpg of hand-calc's prior to the addition of the EZ (now it can vary as much as 2. 0--2. 2 mpg). Do you have Spooled-Up's Fuel Tank Vent Mod ? If not, some of the variances could be due to inconsistent re-fueling of the tank every fill-up. I've noticed a slight difference in foam at two or three places I typically buy my fuel at.

Greg

I do not yet have the vent mod, no. I always fill until the filler neck is full though... so I'm pretty close. I'd say the overhad is within . 5-. 7 MPG, but not perfect.



11 MPG pulling last weekend. A little too much hot rodding!!

 

[Cryotek]

So how does it calculate the DTE(distance to empty)? That is usualy close. I'm sure it uses the fuel gage and MPG but it alwasy seems to be very close. Just wondering why it is close and the MPG varies so much?

 

[Scrappy]

mine w/a TST run on lvl 5 all the time is only off about 1 mpg. , and that is about where it was stock