How it's calculated

[yrralguthrie]

This is sort of a "I think" post. Just asking for comments.
Observing the miles per gallon indicator on the dash it changes easily. Since I have not reset the indication for several thousand miles the mpg indicated changes too much for the calculation to be total miles and total gallons used. So evidently it is calculating the mpg on the last few miles driven. Doing that it may be an accurate indication. This limited calculation is not going to include any stops and idles, which of course decreases the actual total mpg. I'm wondering why it was designed that way.

 

[HSKR R/T]

This is sort of a "I think" post. Just asking for comments.
Observing the miles per gallon indicator on the dash it changes easily. Since I have not reset the indication for several thousand miles the mpg indicated changes too much for the calculation to be total miles and total gallons used. So evidently it is calculating the mpg on the last few miles driven. Doing that it may be an accurate indication. This limited calculation is not going to include any stops and idles, which of course decreases the actual total mpg. I'm wondering why it was designed that way.

It definitely uses idle time in its calculations. Based on injector pulsewidth, and pretty much every engine sensor, as well and engine load and RPM

 

[yrralguthrie]

It definitely uses idle time in its calculations. Based on injector pulsewidth, and pretty much every engine sensor, as well and engine load and RPM

I am not trying to be a horse's butt, but where did you get this information? And a clarification on my part. I meant that the dash mpg indication is not using past idle time. Just that within a few minutes.

 

[DEG]

Doesn't really matter since it's not an accurate calculation anyway. Each fill up I reset the meter on the truck and use Fuelly to manually calculate the milage. The truck never reads the correct MPG not even one time. The truck always reads between .5 and 1.5 mpg better than what I actually got. My last two Rams did exactly the same thing; truck always read higher MPG than actual. I suspect it's an intentional design "flaw".

 

[Richard320]

Let us know if you ever crack the formula. I haven't been able to. How does it calculate mpg and range?

 

[HSKR R/T]

I am not trying to be a horse's butt, but where did you get this information? And a clarification on my part. I meant that the dash mpg indication is not using past idle time. Just that within a few minutes.

Depends on which page you are looking at on the gauge cluster.

 

[yrralguthrie]

Doesn't really matter since it's not an accurate calculation anyway. Each fill up I reset the meter on the truck and use Fuelly to manually calculate the milage. The truck never reads the correct MPG not even one time. The truck always reads between .5 and 1.5 mpg better than what I actually got. My last two Rams did exactly the same thing; truck always read higher MPG than actual. I suspect it's an intentional design "flaw".

I know the dash will not be the same as the actual miles divided by gallons. That's what I said. It is figuring the MPG a different way. I can see this simply because it will change quickly depending on various factors. And I was wondering why it is designed that way. It's just not designed to give total "correct" MPG. And it does matter to me.

 

[yrralguthrie]

Depends on which page you are looking at on the gauge cluster.

I misspoke. I meant to ask where you got the information that the MPG calculation used injector pulse width, almost every engine sensor, engine load, and RPM.

 

[HSKR R/T]

I misspoke. I meant to ask where you got the information that the MPG calculation used injector pulse width, almost every engine sensor, engine load, and RPM.

The computer won't know how much fuel is being sprayed without knowing the pulsewidth.

 

[Fbird]

the "instant" mpg "VIEW you are watching is a simple vacuum gauge setting based on engine load. Nothing more and no where close to actuallity

 

[cerbo]

you must be looking at instant setting? Today I filled at 210 miles and took 10.5 gals. That s 20 MPGs and the truck showed 19.2 ( I alway reset at fillip). Sometimes I could be 1 MPG the other way. So mine is pretty darn close to accurate.
I reset it in the TRIP and on the other MPG indicator one tap up.

 

[yrralguthrie]

No, I am not looking at the instant setting. I am talking about the mpg labeled average.

 

[yrralguthrie]

The computer won't know how much fuel is being sprayed without knowing the pulsewidth.

Again, please tell me where to see that information. And what pulsewidth are you talking about? What do you mean by sprayed? The computer can calculate in many ways.

 

[yrralguthrie]

Doesn't really matter since it's not an accurate calculation anyway. Each fill up I reset the meter on the truck and use Fuelly to manually calculate the milage. The truck never reads the correct MPG not even one time. The truck always reads between .5 and 1.5 mpg better than what I actually got. My last two Rams did exactly the same thing; truck always read higher MPG than actual. I suspect it's an intentional design "flaw".

If I want to understand what is actually happening it does matter. Certainly, the center console mpg calculator does not use the actual total miles driven divided by gallons. But it also is based on some repeatable and consistent parameters. The computer doesn't make mistakes, it does what it is programmed to do.

 

[HSKR R/T]

Again, please tell me where to see that information. And what pulsewidth are you talking about? What do you mean by sprayed? The computer can calculate in many ways.

Well, if you can find the source code for the PCM, I'm sure you could decipher it. If you don't know what pulsewidth I am talking about, or what I mean by being sprayed, it's not worth my time anymore to try and explain.

 

[HSKR R/T]

No, I am not looking at the instant setting. I am talking about the mpg labeled average.

There are multiple displays for MPG. I keep mine in the trip display, which shows miles driven, average MPG, and time since last reset. I reset trip 1 Everytime I fill up. This page uses idle time and the average fmis over the entire time since you reset it last.

There is another display with average MPG and a graph of sorts. This one displayes average MPG over previous 50 miles.

Then there is the display that shows current real time MPG as calculated by computer. How it calculates that depends on if it's in closed or open loop. And many other variables.