# Calculating MPG from VSS and MAF from OBD2

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• 06-16-2009, 12:05 PM
Keith55555
re:Available % of PIDs
Thanks Aberk,that's encouraging.
The last absence I have of information refers to something Bruce Lightner said early on,which is:
"As for other ways of doing this, especially if you don't have a MAF sensor, by knowing the displacement of the engine, and after a simple "calibration" using fuel tank "fill-up" data to find the only unknown, namely the "volumetric efficiency" (VE) of the engine..."
The "simple calibration" and "fill up data" as stated above is what's missing from my understanding. Thanks,Keith
• 06-16-2009, 12:15 PM
Aberk
Quote:

Originally Posted by Keith55555
Thanks Aberk,that's encouraging.
The last absence I have of information refers to something Bruce Lightner said early on,which is:
"As for other ways of doing this, especially if you don't have a MAF sensor, by knowing the displacement of the engine, and after a simple "calibration" using fuel tank "fill-up" data to find the only unknown, namely the "volumetric efficiency" (VE) of the engine..."
The "simple calibration" and "fill up data" as stated above is what's missing from my understanding. Thanks,Keith

The fill up method is when you take your car to the gas station fill it to the top of the neck so that you can see the fuel and can fill it to the exact same spot again. Zero you tripmeter. At your next fill (preferably at the same pump) fill your tank to the same spot and record the tripmeter distance and fuel to fill the car. Divide the distance by the fuel and you have your average MPG. This next part will require fine tuning. Now that you know what you avg MPG is you can adjust the VE in your equation until you are recording the same AVG MPG with your OBD readings as you gathered from the fill method. This all relies heavily on driving habits and such, but the larger the data set you have to use, the more accurate your VE and MPG calculations will be.
• 06-16-2009, 01:46 PM
Keith55555
re:Available % of PIDs
Got it,Aberk. And thanks. The one thing left ,making me scratch my head is; devices like the Kiwi and Scangauge, as far as I know, are 'plug and play' without this 'calibration' using the 'fill up' data. They read cars without MAFs,I assume using the MAP data,etc.
Do you think they are using some form of an average MPG as indicated by the manufacturer in place of the fill up method? Thanks,Again,Keith
• 06-16-2009, 02:08 PM
Aberk
Quote:

Originally Posted by Keith55555
Got it,Aberk. And thanks. The one thing left ,making me scratch my head is; devices like the Kiwi and Scangauge, as far as I know, are 'plug and play' without this 'calibration' using the 'fill up' data. They read cars without MAFs,I assume using the MAP data,etc.
Do you think they are using some form of an average MPG as indicated by the manufacturer in place of the fill up method? Thanks,Again,Keith

They either have an equation that attempts to calculate a VE from other inputs or they assume one. I dont have much experience with the Kiwi, but the SC II is very user involved. The SC II is plug and play for reporting instantaneous MPG. I know the SC II gets more accurate as you use it correctly with fillups and by inputing engine displacement and fuel tank size, so I assume it is logging this data to make this possible.
• 07-08-2009, 03:35 PM
lightner
How to "calibrate" OBD-II derived MPG when using IMAP
Quote:

Originally Posted by Aberk
They either have an equation that attempts to calculate a VE from other inputs or they assume one. I dont have much experience with the Kiwi, but the SC II is very user involved. The SC II is plug and play for reporting instantaneous MPG. I know the SC II gets more accurate as you use it correctly with fillups and by inputing engine displacement and fuel tank size, so I assume it is logging this data to make this possible.

"Plug-and-play" MPG calculation from OBD-II bus parameters is not out of the question for many vehicles---not just MAF-equipped ones. It all depends upon what is available on the OBD bus---from legally mandated OBD-II parameters (i.e., "emissions-related") and the much richer set of manufacturer-specific OBD parameters. Here are some things to consider if a MPG device wanted to make "guesses" about your non-MAF vehicle's engine and gas tank and then "plug-and-play" using IMAP data...without initial user input:

(1) Read the VIN (now a legally mandated OBD-II parameter). With "VIN decoder" firmware (a non-trivial, moving target in itself) one technically can find engine type and displacement, and even fuel tank capacity---as well as paint color!? :-)

(2) Monitor the fuel tank level (now a legally mandated OBD-II parameter). For many vehicles, old and new, this parameter can be read as a manufacturer-specific OBD PID. Knowing (or guessing) the fuel tank capacity, over time (watching for "fill-ups") this can give you what you need to calculate MPG---despite the well known non-linearity of everyone's fuel gauge!

(3) Count the number of O2 sensors and make a guess about the engine displacement and/or fuel tank capacity (e.g., V8 engines have more O2 sensors).

I have some more, but this kind of stuff is potentially valuable, as more and more States---and even the Federal Gov'ment---are looking at a direct "road tax" on our driving. Why? Because per-gallon fuel tax receipts are dropping, as more high-MPG vehicles (e.g., hybrids) hit the road. For example, our friendly government "overlords" need some way to calculate mile-by-mile fuel consumption so that the per-gallon fuel taxes somehow can be refunded to "Big Brother Box" equipped vehicle owners! :-)
• 09-21-2009, 01:07 PM
VegasGuy
Help with formula...
I started with this formula:

MPG = (14.7 * 6.17 * 454 * VSS * 0.621371) / (3600 * MAF / 100)
= 710.7 * VSS / MAF

MPG - miles per gallon
14.7 grams of air to 1 gram of gasoline - ideal air/fuel ratio
6.17 pounds per gallon - density of gasoline
454 grams per pound - conversion
VSS - vehicle speed in kilometers per hour
0.621371 miles per hour/kilometers per hour - conversion
3600 seconds per hour - conversion
MAF - mass air flow rate in 100 grams per second
100 - to correct MAF to give grams per second

Then I realized that Centrafuse return Miles Per Hour for VSS and Cubic Feet Minute for MAF, so I would assumed that in order to correct this formula it would look like this:

MPG = (14.7 * 6.17 * 454 * VSS) / (60 * MAF)
= 686.2891 * VSS / MAF

Eliminates Kilometer conversion
Changes 3600 to 60 since MAF is already calculated to minute rather than second
Eliminates 100 grams calculation

The problem is, the reported results are slightly HIGHER than I would have expected. The original formula without modification produced instant and average milage figures pretty close to what I've seen from other products. The new formula, modiified to take the CentraFuse values into account, produces milage figures slightly higher than before.

Not unreasonable, mind you, but what i would say is at the upper bounds of reliability.

So what am I missing? I think it must be in the MAF calculation, but I'm not sure how to convert between grams per second and feet per minute.

[EDIT]
If I multiply MAF by 0.0807 , that gives me pounds per minute. Then if I multiply by 453.59237, I get grams per minute. Could this be the answer?

Thoughts?

VegasGuy
• 09-21-2009, 01:26 PM
Rooster
I'm not sure what Centrafuse reports, but if it reports MAF in Cubic Feet per minute, that is a volumetric flow not a mass flow. That seems a little strange?
• 09-21-2009, 01:52 PM
VegasGuy
Strange, but true
i just went out and looked, and Centrafuse is definitely reporting CFM. 340CFM at idle to be precise.

My idea about converting cubic feet to pounds and then grams doesn't produce the correct values either.

I'm stumped.
• 09-21-2009, 04:19 PM
split63
Quote:

(2) Monitor the fuel tank level (now a legally mandated OBD-II parameter). For many vehicles, old and new, this parameter can be read as a manufacturer-specific OBD PID.
Has anyone ever come across any details regarding any manufacturer specific PIDs? You would think that after all this time, details would have leaked out.
• 09-21-2009, 06:44 PM
malcom2073
There was a thread on here with some decent info for Ford vehicles, but it disappeared a while back.
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