MPG and range calculation - ZR-1 Net Registry Forums

secondchance · 11-30-2014

Out of curiosity, anyone know how our ECM or BCM calculates these figures? Is this based on injector pulse count? Does fuel level contribute to these calculation?
I deleted secondaries and, as a result, both injectors fire pretty much all the time. And yet, average MPG reads reasonable although I haven't back checked mathematically with mileage vs. amount of fuel to fill up. I was just curious if anyone knew how displayed average MPG is calculated by ECM.

efnfast · 11-30-2014

Good question, I'd like to add to it. How does the car know how much gas is in the tank? For mileage calculations, that needs to be a very accurate number. I suspect more accurate than the fuel gauge can read.

XfireZ51 · 11-30-2014

Doesn't matter whether you have 2 or 1 injector firing, the total PW will be the same. Motor does not use more fuel just because 2 injectors are on. The ECM splits the PW between the 2 injectors. VE is VE. The MPG calc GM uses is a combo of the Inj. PW and pulse counts from the VSS. Don't know about the range calc and the fuel gauge. Looks like a bit of a SWAG for me.

secondchance · 11-30-2014

Quote:

Originally Posted by XfireZ51

Doesn't matter whether you have 2 or 1 injector firing, the total PW will be the same. Motor does not use more fuel just because 2 injectors are on. The ECM splits the PW between the 2 injectors. VE is VE. The MPG calc GM uses is a combo of the Inj. PW and pulse counts from the VSS. Don't know about the range calc and the fuel gauge. Looks like a bit of a SWAG for me.

So the ECM bases calculation on injector pulse width. That being the case, then injector flow rate (lbs/hr) change can affect MPG calculation? Though relatively small, there are flow rate difference between OEM vs. aftermarket injectors.
I think I will start logging miles driven and gallons used and do some calculation to see how accurate MPG calls/display is.

Mystic ZR-1 · 11-30-2014

FWIW:
On my car, the gas mileage shown on the dashboard is always
about 10% low compared to measuring the old fashioned way

XfireZ51 · 11-30-2014

Don't forget the error from tire and wheel size.

11-30-2014	#1
secondchance Join Date: Aug 2006 Location: McLean, VA Posts: 3,712	MPG and range calculation Out of curiosity, anyone know how our ECM or BCM calculates these figures? Is this based on injector pulse count? Does fuel level contribute to these calculation? I deleted secondaries and, as a result, both injectors fire pretty much all the time. And yet, average MPG reads reasonable although I haven't back checked mathematically with mileage vs. amount of fuel to fill up. I was just curious if anyone knew how displayed average MPG is calculated by ECM.

11-30-2014	#2
efnfast Join Date: Nov 2011 Location: Dunbarton NH Posts: 7,375	Re: MPG and range calculation Good question, I'd like to add to it. How does the car know how much gas is in the tank? For mileage calculations, that needs to be a very accurate number. I suspect more accurate than the fuel gauge can read.

11-30-2014	#3
XfireZ51 Join Date: May 2007 Location: Chicagoland, IL Posts: 9,697	Re: MPG and range calculation Doesn't matter whether you have 2 or 1 injector firing, the total PW will be the same. Motor does not use more fuel just because 2 injectors are on. The ECM splits the PW between the 2 injectors. VE is VE. The MPG calc GM uses is a combo of the Inj. PW and pulse counts from the VSS. Don't know about the range calc and the fuel gauge. Looks like a bit of a SWAG for me. __________________ [SIGPIC][/SIGPIC]Former Membership Chairman Former ZR-1 Registry - BOD 1972 Corvette 4speed base Coupe SOLD long time ago 1984 Corvette Z-51/4+3 SOLD 1992 Corvette ZR-1 Aqua/Gray #474 SOLD 1992 Corvette ZR-1 Black Rose/Cognac #458 2014 Honda VFR Interceptor DX

11-30-2014	#5
Mystic ZR-1 Join Date: Jan 2010 Location: Mystic CT Posts: 2,623	Re: MPG and range calculation FWIW: On my car, the gas mileage shown on the dashboard is always about 10% low compared to measuring the old fashioned way

11-30-2014	#6
XfireZ51 Join Date: May 2007 Location: Chicagoland, IL Posts: 9,697	Re: MPG and range calculation Don't forget the error from tire and wheel size. __________________ [SIGPIC][/SIGPIC]Former Membership Chairman Former ZR-1 Registry - BOD 1972 Corvette 4speed base Coupe SOLD long time ago 1984 Corvette Z-51/4+3 SOLD 1992 Corvette ZR-1 Aqua/Gray #474 SOLD 1992 Corvette ZR-1 Black Rose/Cognac #458 2014 Honda VFR Interceptor DX