Overhead MPG Accuracy Fix ?

[SWithrow]

With fuel prices as high as they are, i just take mass transit, or if you need to drive your truck you can turn the over head off or i have found that my compass and temp gauge are fairly accurate.

 

[JStegall]

How about this for cofusion. If I reset while driving on the hiway at 65 MPH with cruise control on it gives me a more favorable reading than if I reset while driving on the hiway 65 MPH with Cruise Control off.

 

[JStegall]

Sorry; new to site.



'04 2500 automatic no mods.

 

[betterthanstock]

There is no such thing as an instant average. If it's instant, it will swing wildly with your foot on the pedal. Pretty useless. They probably do a short duration average, like over the last 10 seconds, or whatever. I think that kind of gage causes drivers to slow down uphill, and speed up downhill, as they try to hold their mileage to some ideal number. It's the long term average (including idle at the drive thru) that you pay for when you fill up.

I know that on my gasser (car brand F) the more fillups you do, the more accurate the gage gets (it's dead on if I don't top it off, and off by about 0. 2 mpg if I do top it off).

I have not tried doing a double reset on the Dodge (double click to reset everything, miles to empty, mpg, hours, etc) at every fillup to see if it improves the accuracy. I'll start doing that.

 

[DrDiesel]

There is no such thing as an instant average. If it's instant, it will swing wildly with your foot on the pedal. Pretty useless. They probably do a short duration average, like over the last 10 seconds, or whatever. I think that kind of gage causes drivers to slow down uphill, and speed up downhill, as they try to hold their mileage to some ideal number. It's the long term average (including idle at the drive thru) that you pay for when you fill up.

I know that on my gasser (car brand F) the more fillups you do, the more accurate the gage gets (it's dead on if I don't top it off, and off by about 0. 2 mpg if I do top it off).

I have not tried doing a double reset on the Dodge (double click to reset everything, miles to empty, mpg, hours, etc) at every fillup to see if it improves the accuracy. I'll start doing that.

OK... Misspoke on that one. I meant instantant MPG. And yes it swings wildly with your foot. It`s real time readout data. But it`s only on my Hybrid. It acts like a vacuum gage would. The scale is from zero to 100 MPG. And yes you do go slow up hill and fast down to keep it in the upper MPG readings. :-laf

 

[brods]

CURRENT AVERAGE

The current average determines what the EST. MTE is. Hence the word EST.

The overhead will never be close to a hand calculated per tank. It`s not designed to display MPG per tank. It only displays the current average.

It takes the current average and multiply`s that by the remaining fuel load as input from the fuel meter/sensor and derives the MTE data.

It`s all related to the current average. The current average has nothing to do with per tank MPG. . :{

"The overhead will never be close to a hand calculated per tank. "???? Please explain how if one resets the overhead when they fill up and then takes the average mpg reading off the overhead at the next fill up (and then resets it), why will this never match hand calculations?



It has been proposed in other threads that there is a limit to how many miles the overhead will average across and if that limit is lower than the typical range on one tank of fuel, than maybe your statement could be true. Do you know for a fact if the overhead has a maximum miles limit? If so what is the number of miles where it starts to rollover?



I hope you really know your stuff so you can explain which inputs the overhead uses to calculate that average mpg. It would be useful to explain why some people get such different hand calculated mpg vs overhead mpg and may help explain why there is such a wide range of mpg between trucks.

 

[rweis]

Are we:



Measureing it with a micrometer,

Marking it with a grease pencil,

Cutting it with an axe?



It's a number. Best guess yours (mine off 25%). Apparently it is not adjustable to each individual truck.



Oh, well. Next idea.



Bob Weis

 

[brods]

Maybe not directly adjustable, but indirectly so by correcting the input that is outside the ideal range.



I calculate mpg every tank. Every time I go to the trouble of checking the overhead it is around 2 mpg high. That leads me to believe the overhead has precision, meaning it can get the same result over and over. Checks of the speedometer using highway mile markers and those "your speed" radar signs seem to confirm my speedo is fairly accurate. So the question is why is the overhead not accurate in my truck, yet it is in others? To understand that one would need to know what inputs it uses to calculate mpg.



Since the "Dr" has been so emphatic with his responses I'm hoping he knows what he is talking about and can enlighten us!

 

[NNovak]

There is no such thing as an instant average. If it's instant, it will swing wildly with your foot on the pedal. Pretty useless. They probably do a short duration average, like over the last 10 seconds, or whatever. I think that kind of gage causes drivers to slow down uphill, and speed up downhill, as they try to hold their mileage to some ideal number. It's the long term average (including idle at the drive thru) that you pay for when you fill up.

I know that on my gasser (car brand F) the more fillups you do, the more accurate the gage gets (it's dead on if I don't top it off, and off by about 0. 2 mpg if I do top it off).

I have not tried doing a double reset on the Dodge (double click to reset everything, miles to empty, mpg, hours, etc) at every fillup to see if it improves the accuracy. I'll start doing that.

So with the information shared here, Could we be making the overhead off by 'topping off' our tanks. I know I try to fill up as much as I can to try to get the same fill up level each time so my hand calculations is accurate. But from what you discribed with your gaser the reading of tank level could be scewed by being 'fully filled'... . I will check my next fill up using the exact same pump and stop fueling when the pump clicks and see if it is more correct.



My main complaint is this technology has been on vehicles since the fuel injection system came out, With computer brain controlling everything now there is no reason this should be off more than . 5 mpg. Specially when every manufacture has them and they are very accurate. So it should not have any issues.

 

[betterthanstock]

So with the information shared here, Could we be making the overhead off by 'topping off' our tanks. I know I try to fill up as much as I can to try to get the same fill up level each time so my hand calculations is accurate. But from what you discribed with your gaser the reading of tank level could be scewed by being 'fully filled'... . I will check my next fill up using the exact same pump and stop fueling when the pump clicks and see if it is more correct.

On my gasser, it shows gallons used, and when I compare to the pump receipt, it's dead on if I don't overfill, and off by about 0. 2 gal when I top it off (that tank is only half the Ram tank). That means when I don't overfill, the mpg gage shows exact miles traveled divided by gallons added, and it's dead on, at least if I let the mpg gage stay on without resetting for the whole tank. It has adaptive learning, and when it says XX miles to empty, it averages my previous mpg over the past 400 miles, to predict my future mpg.

I don't know what exactly the Dodge does, but before my honed injectors, it was almost dead on. I will try double resetting without topping off to see if it improves. And wait until the low fuel light comes on, so the computer can better figure out how much fuel I added to fill it up.

 

[BillDriscoll]

To me it doesnt matter, cause where ever I'm going, it takes diesel fuel to get there. I fill it up when my fuel light comes on. I just tell everyone I'm getting 23 MPG... ... ... ..... lol

BillD

 

[DZLROKET]

I also noticed the part about my speedo being 2 miles optomistic. I chalked it up to the 4. 11 rear end and the 265 tires. .

 

[BTesluck]

I love it..... My Ctd gets 22. 5 mpg regularly then... . LOL. . I still love the truck...



Bob T

 

[Cummins Cowboy]

mine has been at least 3mpg more since day one, maybe the readout is the same as what the early 3rd gens used, you know the trucks that actually got decent mileage

 

[BTesluck]

Just seems to me, with all the technology out there, the high up engineers could have done a better job here. If not,, then dont put it in the vehicle!!



Bob T

 

[DBurdett]

I have a Jeep Grand Cherokee and the Fuel Consumption is always spot on. I can use the fuel consumption and distance travelled to determine exactly how much fuel is required to fill the tank.



Theory as to why our Dodge Truck readouts are off so much.



The sensor registering the diesel fuel delivered to the Injection Pump is probably the same system used in all Chrysler vehicles. However our Cummins Diesels send a proportion of that fuel back to the fuel tank. e. g. some of the fuel from the injection pump is sent back to the tank; also the fuel is used to cool the injectors and I believe some of that is sent back to the tank.

To have an accurate readout of fuel consumption monitoring system would need to monitor flow to the Injection Pump and monitor all the unused fuel flowing back to the tank, and then subtract one from the other.

I suspect that Dodge's cheap solution is to monitor the fuel following to the Injection Pump and use the electronics to subtract 25% assuming that's the amount following back to the tank. (i. e. if only 15% flows back to the tank, then your readout would be off by 10%).

Hence our fuel consumption readout is nothing more than a rough estimate.

 

[brods]

I did some Google searching on this topic the other night and came across this thread. https://www.turbodieselregister.com...head-console-mpg.html#post1833319#post1833319 I posted in it not realizing it was the 2G forum, but I’d bet our 3Gs are similar. It sounds like Bob4X4 knows his stuff, maybe he’ll chime in here.

The ECM rcvs inputs from sensors,and knowing fuel pulse widths,Injector flow #s rpm,map tables etc computes the #s. There are no flow or fuel psi sensors involved.



Bob

My speculation post based on Bob’s info and some of other things I came across.

LOL, several hours of research last night... . and the answer was here all along! Thanks Bob.



So, somewhat oversimplified, the ECM just infers fuel consumption from the lookup tables? It does not monitor pulse width or rail pressure directly to come up with the fuel volume, it just assumes everything is working as its told to? That explains a lot. The overhead comes amazingly close given all the room for compounding errors.



So reasons for the ECM to have incorrect MPG numbers on a stock truck are as follows:



Rail pressure higher or lower than the ECM thinks it is (ie variances in rail pressure sensor accuracy).

Variances in injector flow rates from the ideal injector used for the lookup table data (undersized or partially blocked injector orifices, oversized or worn injector orifices, faster or slower response time than ideal injector).

Leaky injectors.

Speedometer inaccuracies.



Are these correct and are there any others?

 

[AH64ID]

I agree with ^^ posted by brods... what I am curious about it what smarty jr will do to it... my overhead has been fairly accurate... and even thou timing/duration will be changed they are changed in the ECU so it "should" remain as accurate..... the only time mine has been more than 5% off is when I tow at the very beginning of the tank and dont for the last 3/4... which makes me think that even thou I reset it at each fill up it only averages the last 300 or so miles.