Miles Per Gallon - via OBDii code reader
11-28-2005, 02:22 PM
#1
Three Wheelin'
Thread Starter
Miles Per Gallon - via OBDii code reader
Hi guyz n galz,
What with the recent spike in fuel prices and, yes, being a computer geek, I was 'mulling' about how to 'know' your mpg as you drive.
Figured it out! -- based on the MAF (Mass Air Flow).
EXPLANATION
----------------
Since I now know the *weight* of the air going in per min. (based on the reading from the OBDii scanner) am thus able to calculate the *weight* of the fuel that’s being mixed with it (based on the stoichometric value of 14.6:1) per min. That then gives me the amount of GALLONS per min being ‘used’. (Mult. by 60 and I get the gallons per hour)
If I know the speed of the car – which I do – via the speedo – or OBD ii readout – I can know *instantly* the MPG.
Why? because -- miles being driven over time (per hour) compared to gallons of fuel being used over time (per hour).
For instance -- on the LIE – doing 60-70mph – in 6th gear and 2,600rpm -- am seeing MAF values of 3.8 to 4.1 (lbs. of air per minute) – which translates into a healthy 24-26 mpg !!!
Have uploaded a screen-pic (jpg) of the Excel worksheet so you can print out and see for yourself what you're 'getting' inasfar as MPG is concened!
(Those of u who have a code scanner that is.)
Have fun.
G.
What with the recent spike in fuel prices and, yes, being a computer geek, I was 'mulling' about how to 'know' your mpg as you drive.
Figured it out! -- based on the MAF (Mass Air Flow).
EXPLANATION
----------------
Since I now know the *weight* of the air going in per min. (based on the reading from the OBDii scanner) am thus able to calculate the *weight* of the fuel that’s being mixed with it (based on the stoichometric value of 14.6:1) per min. That then gives me the amount of GALLONS per min being ‘used’. (Mult. by 60 and I get the gallons per hour)
If I know the speed of the car – which I do – via the speedo – or OBD ii readout – I can know *instantly* the MPG.
Why? because -- miles being driven over time (per hour) compared to gallons of fuel being used over time (per hour).
For instance -- on the LIE – doing 60-70mph – in 6th gear and 2,600rpm -- am seeing MAF values of 3.8 to 4.1 (lbs. of air per minute) – which translates into a healthy 24-26 mpg !!!
Have uploaded a screen-pic (jpg) of the Excel worksheet so you can print out and see for yourself what you're 'getting' inasfar as MPG is concened!
(Those of u who have a code scanner that is.)
Have fun.
G.
11-28-2005, 02:34 PM
#3
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Is this a standard OBDII scanner or the Bosch Hammer? My $100 Innova scanner doesn't have MAFS or any other stats, just error codes. BTW, very cool logic in figuring this out.
11-28-2005, 02:41 PM
#4
Three Wheelin'
Thread Starter
Originally Posted by epj993
Is this a standard OBDII scanner or the Bosch Hammer? My $100 Innova scanner doesn't have MAFS or any other stats, just error codes. BTW, very cool logic in figuring this out.
I'm surprised your scanner doesn't give u these values. (Are you sure?)
There might be a sub-menu whereby you're able to "View Data" or some such - then it'll ask you to 'view all data?' (or just a subset). It should then 'poll' all the "values" -- (it might take a minute or two) -- that the car's computer is 'sensing/seeing' and _then_ display those values to you.
My scanner is a Matco 9620 -- got it on ebay for $70.
G.
BTW, the 'values' that you should be seeing are e.g. Engine Calc. Load, MAF, Engine RPM, Speed, Oxygen sensor voltages (pre and post cat), Fuel trim, etc. etc. etc.
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I just keep it between 4000 and 5000 RPM in every gear.
11-29-2005, 04:22 PM
#10
Three Wheelin'
Thread Starter
Originally Posted by a17sbn
Nice bit of maths!
The only flaw is the 993's hopelessly inaccurate speedometer!
The only flaw is the 993's hopelessly inaccurate speedometer!
How do I know?
I own a Garmin 296GPSMAP (aviation, auto and marine) unit and routinely have it ON in the car (when I'm heading out to the airport to go flying).
If the car's speedo says 70 ....a flick of my eye to the Garmin display and... yup ...the speed is usually 'seventy something' -- e.g. 70.1 or 70.4 mph or somesuch. I initially put such 'decimal discrepancies' down to slight variations in the amount of foot 'pressure' that I was applying to the accelerator as I moved my eye from the road to the speedo to the unit.
THEN I decided to use CRUISE CONTROL !!!
I gotta say, the speedo on my car is friggin' bang on (up to 90 mph anyways) -- (it's kinda hard to go any faster than 90 on the LIE !!!)
As one of our fellow Rennlisters mentioned -- it would be 'nice' if we could attach an obdii code reader to a car which HAS an 'OBC' so we could 'verify' what I've deduced from the MAF readings.
G.
Oh yeah ...in case anyone thinks I'm BS'g about the airport -- here's the pix that I took (whilst flying) last weekend.
http://i-fly.freeyellow.com/miscell/fulton-co/index.htm
11-29-2005, 04:37 PM
#11
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To be fair, my dissing of 993 speedos was based on my Targa Tiptronic - its speedo was reading ~8% fast when compared with my GPS. I have to say that I never felt like I was travelling as fast the speedo suggested - I don't get that feeling so much with the manual coupe. I need to check how accurate this speedo is.
11-29-2005, 05:07 PM
#12
Three Wheelin'
Thread Starter
Originally Posted by a17sbn
To be fair, my dissing of 993 speedos was based on my Targa Tiptronic - its speedo was reading ~8% fast when compared with my GPS. I have to say that I never felt like I was travelling as fast the speedo suggested - I don't get that feeling so much with the manual coupe. I need to check how accurate this speedo is.
G.
