Help my feeble brain decipher this figure out how to fool my truck's computer. - Ford Powerstroke Diesel Forum
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post #1 of 17 Old 03-08-2014, 01:40 PM Thread Starter
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Help my feeble brain figure out how to fool my truck's computer.

So I'm sitting here, visiting my mother in the hospital, who is sleeping. I got to thinking about my truck's MPG and how the built in computer is so far off. For instance when my dash says I'm getting 21 MPG I'm really getting around 18 MGP. One of the things I like about the Mini Maxx is that I can reprogram my speedo based on the real tire size. When I installed the Mini Maxx I entered the measured tire diameter as 34.25 and it made my speedo more accurate. Not perfect but better.

According to my GPS and a LEO friend's radar, my speedo is about 2 mph off at 60. So when my truck thinks I'm going 60, I'm really going 58. This got me to thinking that not only is my speed off, so is my odometer and therefor so is the calculated MPG. If I drive for an hour at 60 mph, my truck thinks it's gone 60 miles when I've really only gone 58, so the amount of fuel I've used is really to get me 58 miles down the road - not 60. To make this equation easier for my pea brain, let's say my computer says I got 60 MPG. That means that since the truck thinks I went 60 mile, but really only went 58 - my MPG is misstated by 2 MPGs.

See where I'm going with this? Here's my question. Could I, via the Mini Maxx, fool the computer by adjusting my tire size? For instance, what if I plugged in that I'm running 37 inch tires (or whatever) to get my speedo accurate - could I get the MPGs shown on the dash to be accurate as well? If so, how would I calculate what should be input? Any math majors out there? Help me, it bugs me that the computer is so far off.

Last edited by mhutch2; 03-08-2014 at 01:51 PM.
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post #2 of 17 Old 03-08-2014, 05:52 PM
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Maybe you could get it close, but I believe the reason they are off is based on the fuel consumption side of things. The computer doesn't calculate how much fuel you are using correctly. Remember our trucks have a regulated return on the fuel, but there is nothing that measuring the actual amount of fuel going back, its just keeping the pressure correct. I am not sure if the computer is "calculating" fuel useage based on PW, RP, and RPM or on estimated torque (or both). Based on my observations over years, it kinda seems like estimated torque may be at least factored into the equation (which doesn't make sense to me why they would do that). Since we aren't running standard timing with our programmers, the estimated torque will never be accurate. The most I have seen my computer being off was when I ran an early RCD tune on the SCT and it said I was getting 30 to 35 MPG on the highway. Hand calc was no where near that. Even on the same tunes, I have seen the estimated MPG be off by varying amounts depending on the conditions in which I was driving. Bottom line, by altering the tire size you may get it close, but how you are driving it (throttle position, terrian, etc.) will be a variable you can't account for unless you always drive the same route (or you live somewhere flat) and always apply the accelerator the same in every circumstance.

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post #3 of 17 Old 03-08-2014, 05:57 PM
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Two miles per hour doesn't equate to two miles per gallon. If you made 20 mpg and went 60 miles, it would be 3 gallons of diesel. If you went 58 miles and used 3 gallons of diesel, it would be 19.3 mpg.

Hope your momma is doing better

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post #4 of 17 Old 03-08-2014, 06:42 PM Thread Starter
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Thank you Brett! I used 60 MPG just to make it easier on my brain and not have to convert anything to a common dominator.

B585 - You lost me somewhere around "Remember our trucks have a regulated return on the fuel...." So what you're saying is the computer doesn't know how much fuel it's used? How does it come up with a MPG if it doesn't factor fuel used vs. miles traveled? Way beyond me!!!!

I told you I was simple minded.
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post #5 of 17 Old 03-08-2014, 08:41 PM
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Originally Posted by mhutch2 View Post
Thank you Brett! I used 60 MPG just to make it easier on my brain and not have to convert anything to a common dominator.

B585 - You lost me somewhere around "Remember our trucks have a regulated return on the fuel...." So what you're saying is the computer doesn't know how much fuel it's used? How does it come up with a MPG if it doesn't factor fuel used vs. miles traveled? Way beyond me!!!!

I told you I was simple minded.
I missed it the first time I read your post about being in the hospital so definitely, I hope she is getting better.

I am saying I don't think the computer ACCURATELY knows how much fuel it is using. It does calculate fuel consumption and I am not sure exactly what formula it is using. The whole thing about the return is...the computer knows fuel is coming in. On a 6.4, the computer doesn't even know the pressure of the fuel past the LP pump until it is past the HPFP (it does know the rail pressure after the HPFP). The problem is not all of that fuel that goes through the LP system is going to the injectors....some of that fuel is returned. So the computer has to "calculate" fuel consumption instead of actually "measuring" it. I believe the method it uses to do the calculation is prone to being inaccurate (especially when you are not running stock tunes). One of the things that has also puzzled me is why Ford didn't add a feedback calculation to at allow the computer to get more accurate over time. Here is an example of a feedback factor. You are getting 16 MPG but the lie-o-meter says you are getting 20 MPG. If you drive 400 miles, the computer would expect that you burnt 20 gallons, but in reality you burnt 25 gallons. The computer could use the size of the tank and what the gauge says is left in the tank so it should easily be able to calculate that you burnt 5 more gallons than it would have predicted over that 400 miles. It could take that info and drop the MPG to a more accurate number. Hope that makes sense.

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post #6 of 17 Old 03-08-2014, 11:07 PM
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The thing I don't get is my wife's 150 EB is within .1 gallon on the fuel used vs the pump. Mine is usually a couple gallons lower than the pump. Why the difference unless it is the return like your saying. It is annoying. On my duramax you could count on the amount used and I planned accordingly but ford and dodge haven't nailed it down yet.

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post #7 of 17 Old 03-09-2014, 06:54 AM Thread Starter
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Every vehicle I've owned till now was within half a mile accuracy on the MPG. I'm disappointed that I'm hearing mine can't be corrected. I thought I was onto something.
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post #8 of 17 Old 03-09-2014, 04:14 PM
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Im not much help with the math but Ido hope your mother gets well.

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post #9 of 17 Old 03-09-2014, 05:16 PM
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Originally Posted by mhutch2 View Post
Every vehicle I've owned till now was within half a mile accuracy on the MPG. I'm disappointed that I'm hearing mine can't be corrected. I thought I was onto something.
If you drive in a similar fashion, terrain, load, and tune you might be able to consistently be within 1 MPG after compensating with tire size...maybe even 0.5

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post #10 of 17 Old 03-09-2014, 06:35 PM Thread Starter
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OK, let's assume all things being equal. What would be the equation to calculate the tire size to input. Here are the factors I know:

Actual tire size is 275/65R20 which is 34.25 diameter
Speedo is approx. 2 mph off at 60 mph (shows faster than actual speed)

Can anyone tell me what diameter to input to correct speedo? If so, I'll try that and report back if the MPGs is any closer to reality.

Thanks all and thanks for the good wishes for my mother. Unfortunately it's just the beginning of the end. She's 86, severe diabetic and suffering from acute congestive heart failure. Not much we can do for her except make her comfortable.
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