TurboTed

As for what I`m driving, it`s a CTD....



That's a good thing!!

packard8

As I recall, one of the resident Dodge techs here explained that the overhead uses the Throttle Position Sensor values and the number of injector pulses plus the Odometer reading to approximate an average MPG since the last time it was reset.

If this is indeed the case, it would be independent of how the tank is filled (topped off or not). Mine has been pretty accurate compared to hand calculating over the past 9500 miles.

DrDiesel

I don`t think so...
The only thing injector driver info is used for would be related to misfire info. That data is volatile memory and after 256 counts per event with a maximum of about 10 events (it`s a guess) it cycle`s back to zero(dumps out) and starts over. The data bank to use injector pulse info for fuel mileage would require to much memory storage. The new PCM`s have a higher baud rate and higher voltage, but memory remains limited. If a code has triggered the freeze frame data recorder you can get up to 30 frame before and after the code was set, but beyond that range it`s not capable

BigHornCTD

Exactly what I have been getting at all along Doc.

Your first post in this thread said the overheads are accurate and give the correct info. And some of us intepreted your "at that time" statement to mean instantaneous, which I think has since been clarified.

Now you are saying that it may be inaccurate because of the variables listed, which I (and likely most everyone else) completely agree, and was the point of the whole discussion. As far as not overfilling the tank, you suggested that as a way to remedy inaccuracies, however now it sounds like using the standard 1-click method may not work.

I know from reading pages of topics on this feature, that a lot of folks would like to know more about how it really works. That is why we are here, and why I ask questions. I would like some more information on the subject if you have any sources. Any further discussion or theories are pointless until someone can provide reliable information.

DrDiesel

I`ll see what I can dig up, beside`s Dodge propaganda

White97GTVert

The overhead MPH calculation in our truck DOES NOT CARE how much fuel is in the tank. Its uses your throttle position, boost pressure, and compares that to tables to calculate fuel flow based on RPM and SPEED. It does not take into account how much fuel you managed to cram into your fuel tank.

Rocky5.9

I have to go with DrDiesel right now... when your truck tells you it's getting evleventy miles per gallon, it is only gettin that at the moment. Your own foot determines how the rest of your tank will play out. Fill up as much as your tank will hold, reset your trip odometer, then when you fill up again take the miles on the odometer and divide them by the gallons you put in. No false advertising there.

ramtd02

Higher voltage??? Last time i checked the ECM and PCM's still use 12V dropped to 5V for sensors, and as a minimum can run on 5V. But who knows i am no Doctor, just a low life engineer....LOL

truk

if it measures off the tank then idleing will kill your mpg right ?

packard8

That’s pretty much what I’ve seen posted by some knowledgeable sources. I said earlier that injection cycle data were also factored into the calculation, but it may have been injector duration data instead.

There is also apparently different programming in the earlier trucks compared to the 2006, and even among the 2006’s depending on the date of the programming. After having my truck reflashed for the “hard start TSB”, the MPG reading seems to change more rapidly than it did before the reflash.

At any rate, I don’t believe that the overhead reading is influenced by how the tank is filled or that the fuel gauge float is used to calculate MPG…(tho I could be wrong….lol) .

BigHornCTD

Heaven forbid. Dodge can't possibly understand how their system works or what it is intended to do.

But nevertheless, I am curious.

BigHornCTD

This is what I am curious about. If I refill my tank and reset the overhead, it starts with the last known reading. If I let the truck idle, the reading rapidly drops. But because there is some amount of fuel above the tank sender, it does not think that any fuel is being used. So why would the reading drop? According to the sender, I am not using fuel and I am not moving so my economy reading should at least stay the same until I drive. As soon as I start driving, my economy reading should then rapidly go up for a period of time, because according to the sender I am still not using fuel but I am now logging miles.

So to me it seems like there is other input or programming involved to give the truck an estimate of what it thinks it's economy is. But who knows, now all I am doing is speculating, . That's why I would like some more information to satisy my curiousity, but I have done some digging and asked questions without finding out anything. So I will live with my pencil and paper, it doesn't require any VooDoo Hoo Doo.

packard8

Exactly.... when your engine is running and the tires aren't turning you are getting ZERO MPG....

CalDodge

I just got back from a 245 mile trip to the desert and got 27.2 mpg on the overhead. So your telling me this isn't bull.

packard8

Yup, I’d like to hear it “from the horse’s mouth” too. A while back one of our Dodge tech members answered the question. As I recall the computer reads several data inputs including TPS, boost, *something* from the injectors or IP, speed, odo, etc (but I don’t think the fuel gauge or sender are involved).

Of course the most accurate approach would be a flow meter and certified odometer, but that’s not the case. At any rate, my overhead seems reasonably close to reality when compared to hand calculations over the period I’ve owned the truck (9500 miles).