How it's calculated

[cerbo]

mpg-calculation.84587

So I don t think we have a clear answer to the calculation questions. I m surprised. Mine is so close that it works for me.

 

[John Jensen]

I meant this post as information gatheiring on my part, I did not anticipate people getting annoyed and hateful. I'm NOT talking about you.

For something so basic you made it comprehensive and now accuse people.
I don't think people got annoyed or were hateful. It reads more like you were annoyed with their responses. I can understand your desire to know how the system works but don't understand the depth you wanted. If you got all that you wanted, then what would you do with it? You can't change it, why not just accept it like the rest of us?

 

[yrralguthrie]

John, all I asked for was information. You are welcome to accept, or decline to answer, that is your right and no one else's. I respect your rights. If you don't want to answer, just don't, but I will be the judge of what I want to learn and what I want to do with it. I would like to know how the mpg display operates. I'm curious, and I don't see any harm in that, I don't want to change it, and what others know is not going to shape my thinking.

 

[yrralguthrie]

I am genuinely surprised at the angst I've created by asking about the mpg calculation. I'm not and have not questioned anyone's answers. Any information can be useful. Anyone's information is welcome. If I ask for an explanation I'm not denigrating you. I have asked for explanations when I didn't understand. I understand the center display is not the same as the mpg calculated by hand at each fill-up. I don't understand about pulse width and spraying. I certainly could not understand or change the PCM programming. I don't understand why virtually all center displays of mpg are not closely similar to the actual mpg calculated. On any car or truck. Are all car computers programmed to make the same errors? Not trying to get kicked off the forum.

 

[cerbo]

The indicators for MPG can be accurate . Mine is only 1 MPG off (sometimes) . 7500 miles so far.
Ran my Mustang 700 miles and it was spot on.
My wife 24 Mazda usually reads about 3 MPG high.
I remember I had a Subaru and there was a way into the system that let you adjust which way it was OFF, that was a while ago so I can t remember the details.
I don t see your looking for a better explanation as an issue. Sometimes things read differently than intended.

No need to worry about being kicked off the forum for that.

 

[John Jensen]

John, all I asked for was information. You are welcome to accept, or decline to answer, that is your right and no one else's. I respect your rights. If you don't want to answer, just don't, but I will be the judge of what I want to learn and what I want to do with it. I would like to know how the mpg display operates. I'm curious, and I don't see any harm in that, I don't want to change it, and what others know is not going to shape my thinking.

You did a bit more than ask for information. You posted (#18) that people were getting annoyed and hateful. It's that post that I was replying to.

 

[HSKR R/T]

I am genuinely surprised at the angst I've created by asking about the mpg calculation. I'm not and have not questioned anyone's answers. Any information can be useful. Anyone's information is welcome. If I ask for an explanation I'm not denigrating you. I have asked for explanations when I didn't understand. I understand the center display is not the same as the mpg calculated by hand at each fill-up. I don't understand about pulse width and spraying. I certainly could not understand or change the PCM programming. I don't understand why virtually all center displays of mpg are not closely similar to the actual mpg calculated. On any car or truck. Are all car computers programmed to make the same errors? Not trying to get kicked off the forum.

I took your questions as questioning my information. I apologize for the misunderstanding.

Pulsewidth is basically the amount of time the injectors spray fuel into the cylinder. The computers adjusts how much fuel is sprayed through the fuel injectors using the pulsewidth. If more fuel is needed, the injectors spray longer, meaning higher pulsewidth.

There really isn't an easy answer to how the calculation works because it varies based on operating conditions. If you ever get into engine tuning, there are multiple tables used to adjust fuel that use almost every sensor on the engine. From coolant temp, to air temp, throttle position, manifold vacuum, oxygen sensor input, knock sensors..... There are different tables for open loop operation(engine warm up and full throttle) or closed loop(part throttle after engine warms up). This i partially why the average displayed on dash(and it depends on which screen you are one) doesn't always match the hand calculated mileage.

And even hand calculating isn't exact as different fuel pumps click off at different times, and have a tolerance window for how the pumps calculate the fuel being pumping into your tank. Getting fuel in the morning when it's colder will result in a different amount of fuel than if you get fuel in afternoon when it's hot. How deep the fuel tanks and gas station are buried also makes a difference.

My display is usually about 1-1.5mph higher than hand calculated. But I've been as close as .5mpg off. Making longer trips and steady speed will result in the difference being smaller. Lots of short trips, the difference will be higher.

 

[GMetal]

The ECU knows the load-MAP sensor
It knows the amount of fuel being delivered by injector size and how long it flows this is the Injector Pulse Width
It knows the time the engine has been running
It knows the mileage the vehicle is driven.
It knows the amount of fuel in the tank and its capacity.
All these factors are what it uses to determine MPG its never 100% accurate but close enough for most consumers

 

[jamiehughes]

I took your questions as questioning my information. I apologize for the misunderstanding.

Pulsewidth is basically the amount of time the injectors spray fuel into the cylinder. The computers adjusts how much fuel is sprayed through the fuel injectors using the pulsewidth. If more fuel is needed, the injectors spray longer, meaning higher pulsewidth.

There really isn't an easy answer to how the calculation works because it varies based on operating conditions. If you ever get into engine tuning, there are multiple tables used to adjust fuel that use almost every sensor on the engine. From coolant temp, to air temp, throttle position, manifold vacuum, oxygen sensor input, knock sensors..... There are different tables for open loop operation(engine warm up and full throttle) or closed loop(part throttle after engine warms up). This i partially why the average displayed on dash(and it depends on which screen you are one) doesn't always match the hand calculated mileage.

And even hand calculating isn't exact as different fuel pumps click off at different times, and have a tolerance window for how the pumps calculate the fuel being pumping into your tank. Getting fuel in the morning when it's colder will result in a different amount of fuel than if you get fuel in afternoon when it's hot. How deep the fuel tanks and gas station are buried also makes a difference.

My display is usually about 1-1.5mph higher than hand calculated. But I've been as close as .5mpg off. Making longer trips and steady speed will result in the difference being smaller. Lots of short trips, the difference will be higher.

great answer buddy.. thank you for that information..