How does the computer figure miles per gallon?
Thread Starter
Safety Car
Joined: Jun 2004
Posts: 4,353
Likes: 22
From: Bella Vista Ca
How does the computer figure miles per gallon?
How does the computer figure the car's actual miles per gallon? Does it actual monitor the amount of fuel flow over distance..... Do after market fuel components/ systems change things 
Racer
Joined: Mar 2006
Posts: 464
Likes: 0
From: silverdale washington
From watching what changes it I would guess it uses rpm, tps, and speed, and some constant programmed from the factory that is incapable of correcting for any mods. Use to be 28mpg on highway, now w/ 3:73 and an m-12 It seems to think I get 38mpg at about 70 cruise. More like 25mpg or so.
Race Director
Joined: Jul 2004
Posts: 14,593
Likes: 219
From: Ohio
If you start with a full tank and reset the mpg, you'll get the most accurate reading.
The computer probably calculates from the fuel gauge settings and the odometer. Not a real difficult calculation.
The computer probably calculates from the fuel gauge settings and the odometer. Not a real difficult calculation.
Burning Brakes
Joined: Nov 2002
Posts: 807
Likes: 21
From: Los Altos CA
I believe it uses actual miles travelled and the injector pulse width. This is the way it was on the C4 and other cars I have owned. With the number of pulses per second, the pulse width, the capacity of the injectors and the number of injectors, it can do a fairly good job of estimating the fuel flow rate.
If you change injectors, it will use the wrong fuel flow per injector. Mine reads about 8% high and my C4 was 33% high until I replaced the injectors and then it became about 10% low. I think this can all be calibrated in the pcm.
If you change injectors, it will use the wrong fuel flow per injector. Mine reads about 8% high and my C4 was 33% high until I replaced the injectors and then it became about 10% low. I think this can all be calibrated in the pcm.
Burning Brakes
Joined: Feb 2004
Posts: 1,024
Likes: 108
From: Ocala, Fl. State of Confusion
I would have guessed that there was some kind of flow meter in the fuel line used in connection with the speedo. My '83 El Dorado had this mpg readout and it was very accurate, as is the vette.
Race Director
Joined: Mar 2000
Posts: 17,602
Likes: 20
From: Palm City Florida
Originally Posted by white90conv
I believe it uses actual miles travelled and the injector pulse width. This is the way it was on the C4 and other cars I have owned. With the number of pulses per second, the pulse width, the capacity of the injectors and the number of injectors, it can do a fairly good job of estimating the fuel flow rate.
If you change injectors, it will use the wrong fuel flow per injector. Mine reads about 8% high and my C4 was 33% high until I replaced the injectors and then it became about 10% low. I think this can all be calibrated in the pcm.
If you change injectors, it will use the wrong fuel flow per injector. Mine reads about 8% high and my C4 was 33% high until I replaced the injectors and then it became about 10% low. I think this can all be calibrated in the pcm.
I reprogrammed my IPW (A few % Richer)... So I KNOW my MPG reading is off...
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Burning Brakes
Joined: Aug 2006
Posts: 1,217
Likes: 5
From: Stark County Ohio
My stock 2003 LS1 calculates 25MPG highway while it is actually getting 30MPG highway, according to calculations the old fashioned way – Fill-up-to-Fill-up. (I’ve never calculated mileage around town)
With two gas tanks, etc the calculation program has lots of room for error.
With two gas tanks, etc the calculation program has lots of room for error.
Le Mans Master
Joined: Sep 2004
Posts: 5,707
Likes: 4
From: Los Angeles
Originally Posted by timemender
My stock 2003 LS1 calculates 25MPG highway while it is actually getting 30MPG highway, according to calculations the old fashioned way – Fill-up-to-Fill-up. (I’ve never calculated mileage around town)
With two gas tanks, etc the calculation program has lots of room for error.
With two gas tanks, etc the calculation program has lots of room for error.
Le Mans Master
Joined: Sep 2004
Posts: 5,707
Likes: 4
From: Los Angeles
Originally Posted by Face
I noticed people who upgrade thier injectors, thier calculations are way off. So I'd be willing to bet injector pulse would be one of the factors.
Le Mans Master
Joined: Jan 2001
Posts: 5,785
Likes: 6
From: Lafayette Colorado
Originally Posted by Face
I noticed people who upgrade thier injectors, thier calculations are way off. So I'd be willing to bet injector pulse would be one of the factors.
Keith
Safety Car
Joined: Oct 2000
Posts: 4,949
Likes: 10
From: Monticello MN
Originally Posted by Face
I noticed people who upgrade thier injectors, thier calculations are way off. So I'd be willing to bet injector pulse would be one of the factors.
I can vouch for that.
Ed
Drifting
Joined: Mar 2005
Posts: 1,737
Likes: 0
From: Northern VA
Originally Posted by Face
I noticed people who upgrade thier injectors, thier calculations are way off. So I'd be willing to bet injector pulse would be one of the factors.
Tech Contributor
Joined: Dec 1999
Posts: 32,910
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Its fuel flow (using injector pulde with "given a known volume of fuel that the injector will flow") verses distance travled.
Turn your DIC to display "Instant MPG" while your doing 60-70 MPH and throw it in to nuteral and coast with the engine at idle. The instant MPG will run up to 99.00 MPG and slowly drop as your speed decreases. The only variables that are changing is speed (tire rotation) and injector pulse width (engine idling at 750 RPM).
BC
Turn your DIC to display "Instant MPG" while your doing 60-70 MPH and throw it in to nuteral and coast with the engine at idle. The instant MPG will run up to 99.00 MPG and slowly drop as your speed decreases. The only variables that are changing is speed (tire rotation) and injector pulse width (engine idling at 750 RPM).
BC
Last edited by Bill Curlee; Oct 26, 2006 at 02:58 PM.
Safety Car
Joined: Sep 2003
Posts: 3,558
Likes: 2
From: Cape Coral FL
Originally Posted by Bill Curlee
Its fuel flow (using injector pulde with "given a known volume of fuel that the injector will flow") verses distance travled.
Turn your DIC to display "Instant MPG" while your doing 60-70 MPH and throw it in to nuteral and coast with the engine at idle. The instant MPG will run up to 99.00 MPG and slowly drop as your speed decreases. The only variables that are changing is speed (tire rotation) and injector pulse width (engine idling at 750 RPM).
BC
Turn your DIC to display "Instant MPG" while your doing 60-70 MPH and throw it in to nuteral and coast with the engine at idle. The instant MPG will run up to 99.00 MPG and slowly drop as your speed decreases. The only variables that are changing is speed (tire rotation) and injector pulse width (engine idling at 750 RPM).
BC
I think this is how it's figured. You gave the solution by stating the factor as ("injector pulse with "given a known volume of fuel that the injector will flow"") is actually a table. It's the Injector Flow Rate table, and it's calibrated in lbs./hr by MAP or Manifold Air Pressure.So based on the MAP which is a very close approximation of the load on the engine, it has a lookup value for the actual amount of fuel being used at any time. It then could use that figure coupled with the speed to compute the amount of fuel being used. I think it uses the instant value and caches that, then averages that value continuously over a period of time and samples to determine the average fuel used.
