Was driving the other day and noticed my overhead console display going haywire. I've had it on the fuel economy display lately & was pleased to see my mileage creep up with the warmer temps (got up to 19.7 mpg). Suddenly the numbers started to increase rapidly, about 2 tenths every second, then more like 2 mpg per second. The number kept on spooling up until it hit 99.9 then it stopped there. While it was spooling up I scrolled through the different displays & saw the miles to empty reading spooling real fast too. A few moments after it hit 99.0 I did a reset while I continued driving: it came back down to 2 mpg and then started to increase again but at a much slower rate, like a tenth every 10 seconds or so. It topped out at 27.5, then started to drop with stops & hills, eventually hovering around 24.3 to 25.2 until I parked the vehicle. Today I started it back up for the first time since the above incident. The display was right where it had left off at about 25.1. But this morning was in the 20s vice the 70s of the other day, and the MPG started to drop as expected, especially in the slow stop & go city driving. It dropped to 23.0 but went back up to about 23.2 after a short run on the highway.Naturally the present indicated mpg is inaccurate, but before the incident I feel it was functioning properly. Can't verify that until I fix the fuel tank leak that happens when I top it off (waiting for warmer weather). I guess that some connection went flakey, perhaps a fuel input from the ECM. And I guess that when I reset the MPG computer it should have reset to zero. I plan on resetting it again while stopped & see what happens.I'm not really clear on what the fuel economy reading is indicating. In the 2001 FSM it explains two economy displays; average (over past 30 gals) & instantaneous. Not sure what my '99 is displays. I read the owners manual & it wasn't very helpful.Any comments appreciated,Joe in St Louis

Well all data is calculated simply... MPH / GPH = MPG So if your traveling 55 MPH and the engine is consuming 3.0 GPH so... 55 MPH / 3.00 GPH = 18.3 MPG Then what the overhead does is buffer the information and keep averaging the data out. But if you want to reverse calculate you can too... GPH x MPG = MPH MPH / MPG = GPH

Well all data is calculated simply... MPH / GPH = MPG So if your traveling 55 MPH and the engine is consuming 3.0 GPH so... 55 MPH / 3.00 GPH = 18.3 MPG Then what the overhead does is buffer the information and keep averaging the data out. But if you want to reverse calculate you can too... GPH x MPG = MPH MPH / MPG = GPH

How does it calc gph mike ?? Based on some signal to the injector ? But then what happens when u go bigger etc ?

How does it calc gph mike ?? Based on some signal to the injector ? But then what happens when u go bigger etc ?

That's where the stock overhead computer falls short quick...

Like Ultra Gauge and Scan Gauge both have a offset feature to calibrate to offset changes in the fuel system like larger injectors, timings, etc... Then This figure is used in conjuction with the GPH signal from the ECM to correct flows.

Then UG & SG both have a calibration for speed so then you can dial in for over sized tires and such...

So now both numbers are trued back up giving better accuracy to the MPG display compared to the stock overhead computer...

((MPH * Offset %) + MPH) / ((GPH * Offset %) + GPH) = Corrected MPG

Now this is something stock overhead can't do...

So like mine has a GPH offset of 24% and has a MPH offset of 1%... Which gives true number from the SG which my hand math tends be lower because of the 1% difference in mileage.

Like my Mom's 96 (Running Mopar Performance PCM) has a 233% GPH offset and 0% MPH offset. So SG / UG is a good tool if calibrated properly...

Thanks Mike.

I understand the basic math. What is unclear is what the buffer is doing. If the computer is trying to average MPG over the last 30 gal burned, then it would make sense that the display is changing rapidly after a reset since it could only average the MPG over the fuel burned following the reset.

I reset the display again yesterday afternoon. This time I did it with the engine idling while in park. Again the display for "avg eco" reset to 2 MPG. I drove the short 1 mile drive from the parking lot at work to get onto the highway. The display would update about every 5 seconds gaining a tenth each time until it reached 21.0 or so, where it hovered until I exited the highway back onto city streets. Sitting at stop lights the number displayed would decrease slightly, but the updates were closer to 1 minute rather than 5 seconds. This morning's run into work was a continuation of yesterday's performance - lower economy when cold and in stop & go traffic, improved economy at steady state in O/D with TQ lock-up.

I'd say it seems to be functioning correctly although I believe the reading is a bit optimistic, perhaps by 2 mpg. And I assume the frequent display updates are normal since I've only burned 4 gal or so of fuel since the last reset. I'd like to know if the operation I've described sounds normal, particularly the frequent updating of the display immediately following a reset. If anyone has access to a FSM from 1999 or earlier I would appreciate hearing how the system is described, since it appears (as per the 2001 FSM available here) that the trip computer went through an update since mine was built.

Thanks for your patience,

Joe in St Louis

--- Update to the previous post...

That's where the stock overhead computer falls short quick...

Like Ultra Gauge and Scan Gauge both have a offset feature to calibrate to offset changes in the fuel system like larger injectors, timings, etc... Then This figure is used in conjuction with the GPH signal from the ECM to correct flows.

Then UG & SG both have a calibration for speed so then you can dial in for over sized tires and such...

So Mike, how do you come up with the offset figures? How did you arrive at 24% for your GPH offset?

Thanks,

Joe in St Louis

Going to mark my answers in RED.

Thanks Mike.

I understand the basic math. What is unclear is what the buffer is doing. If the computer is trying to average MPG over the last 30 gal burned, then it would make sense that the display is changing rapidly after a reset since it could only average the MPG over the fuel burned following the reset.

What I think is happening is the overhead computer is getting a strange signal from the CCD network of 9,999 MPG instant so this would drive the AVG MPG up rapidily.

I reset the display again yesterday afternoon. This time I did it with the engine idling while in park. Again the display for "avg eco" reset to 2 MPG. I drove the short 1 mile drive from the parking lot at work to get onto the highway. The display would update about every 5 seconds gaining a tenth each time until it reached 21.0 or so, where it hovered until I exited the highway back onto city streets. Sitting at stop lights the number displayed would decrease slightly, but the updates were closer to 1 minute rather than 5 seconds. This morning's run into work was a continuation of yesterday's performance - lower economy when cold and in stop & go traffic, improved economy at steady state in O/D with TQ lock-up.

Typical there... I show a reduction in MPG when cold till about 170*F worth of coolant then it starts to rise.

I'd say it seems to be functioning correctly although I believe the reading is a bit optimistic, perhaps by 2 mpg. And I assume the frequent display updates are normal since I've only burned 4 gal or so of fuel since the last reset. I'd like to know if the operation I've described sounds normal, particularly the frequent updating of the display immediately following a reset. If anyone has access to a FSM from 1999 or earlier I would appreciate hearing how the system is described, since it appears (as per the 2001 FSM available here) that the trip computer went through an update since mine was built.

Thanks for your patience,

Joe in St Louis

Well the stock system just listens to the GPH signal from the ECM and process against the MPH signal from the ABS computer then buffers the numbers for a average. But since the overhead computer doesn't display a INSTANT MPG value you can't tell if the signal is crewy or not. The only tool that could is a UG/SGII or any tools with live data.

--- Update to the previous post...

So Mike, how do you come up with the offset figures? How did you arrive at 24% for your GPH offset?

Thanks,

Joe in St Louis

Because during the calibrate fillup process you would fill the truck at the fuel station and then drive till about 1/4 tank and return back to the same fuel station and pump. Then fill again. During this fill the pump would show the total amount of fuel pump into your fuel tank and the ScanGauge II would show the amount fuel consumed. So knowing that both numbers are off the offset is adjusted to match the pump. So say SGII displays 22.0 Gallons and I pumped 27.2 Gallons so I would end up increasing the offset till the SGII match the fuel pump which would display 24%. So the next time you pull up to the pump the gallons should be right there close when it kick the pump handle.

NOTE: You will never get it exactly perfect. Fuel station pumps have a margin of error as well.

So that is the short fall of the stock overhead it assumes that it will remain a stock truck forever and you never change tires, rims, engine fueling system, etc...

So like mine has a GPH offset of 24% and has a MPH offset of 1%... Which gives true number from the SG which my hand math tends be lower because of the 1% difference in mileage.

I'm running 43% offset. I wonder why my truck would be so much more than yours considering my mods are the same as yours aside from the injectors. Why doesn't the quote that I pasted not show that's it's a quote?

Like on my Mom's 96 Dodge Ram 1500 V8 5.9L the offset is 233%... It shows at 0% offset of 55 MPG... Which I know isn't true... So as for why the offset differences? I don't know really... Difference in electronics? Orignal in stock form I was only 9.6% offset... Now with all the mods I'm at 24%...

There IS an "instant" MPG mode for the overhead on my 01.5. I don't find it useful since it swings wildly with the slightest grade. The overhead average MPG has been 100% right on with my stock truck, compared with hand calculated. I'm going to miss it when I get my Christmas presents installed. I shame we can't adjust the overhead... like NOT being able to is going to discourage us from modding the truck.Russ

Well I filled up again today and I had to bump the offset to 50%. My last 577 miles netted me 7.5 mpg:stuned::banghead:. I'd say about 40% of that was towing about 13k pounds, 40% towing 4k pounds (empty livestock trailer) and the remaining 20% just the truck. Something HAS to be wrong with this beast. There's no fuel leaking on the ground either. One brake was 30* warmer than the rest so I guess it's dragging a little bit. Is that enough? Or could the vp44 be suffering from it's days before the Airdog? Or could it be bad injectors? EGT's run about 800 climbing small hills and 650-700 on level going. Outside temps around 5* F. Sorry this is somewhat off topic.

Well I filled up again today and I had to bump the offset to 50%. My last 577 miles netted me 7.5 mpg:stuned::banghead:. I'd say about 40% of that was towing about 13k pounds, 40% towing 4k pounds (empty livestock trailer) and the remaining 20% just the truck. Something HAS to be wrong with this beast. There's no fuel leaking on the ground either. One brake was 30* warmer than the rest so I guess it's dragging a little bit. Is that enough? Or could the vp44 be suffering from it's days before the Airdog? Or could it be bad injectors? EGT's run about 800 climbing small hills and 650-700 on level going. Outside temps around 5* F. Sorry this is somewhat off topic.

Well I know from personal experince cold air is not your friend 24V Cummins burns much better with warmer air say 100-140*F IAT temp so at 5*F I bet the IAT is about 45*F. At those conditions and that amount of weight yes 7.5 MPG. Now if you were to jump into the IAT mod I think you can gain back some MPG's... Brake drag does come on rather slowly and eat away at you MPG numbers quickly... I would just brake down all 4 wheels and disassemble and clean all brake parts really good and be sure that all part fit properly without binding up. Just my trip down to Boise, ID with my Jayco trailer (no IAT mod) I got got barely 9.9 MPG. With outside temps around 10*F to 20*F for the day. Kept my speed to 55-60 MPH and was seeing the same kind of pyrometer temp except when I climbed over Midvale grade 6% grade at 60 MPH I was looking at 1,100*F and 32 PSI of boost. After talking to CajFlynn he suggested I back out and limit to 20 PSI boost at anytime this reduces the chance of driveline failure and slight gain back to MPG's.

I need an IAT fooler.

I've been running with my iat at 145. By watching my scangauge I have to imagine that I'm seeing an improvement. I try switching between normal and fooled temp and the mpg reading sometimes goes up and sometimes it goes down when it's fooled.