Trip Computer MPG Accuracy?

[Bernie]

My 2006 CTD has about 16k miles on it. My overhead console trip computer MPG calculation is a monument to inaccuracy. It is optimistic by 1. 5 to 3 MPG.



Last tank read 18. 0 MPG average. When I filled it and did the math the actual was 15. 6



Is this normal? Is there anything that can be done to make it accurate?



I always assumed with a diesel it would be very easy to make an accurate MPG gauge. The computer should just count the number of injector pulses and compare it to wheel speed to come up with an accurate MPG reading. Is my thinking wrong, or did Dodge cut a corner and try to estimate the MPG based on something else? If so, what?

 

[RWeiss]

Mine is about the same 1. 0 to 1. 5 on the high side every fill up... . dealer said its the way I drive..... thats messing up its computations... not consistant with my foot I guess ... go figure!

 

[DZink24]

Mine actually reads about . 3 on the low side. I have shown 17. 7 and 18. 3 on consecutive 600+ mile tanks, and when measured, 18 and 18. 7. I have no electronic mods. That second tank was my first full tank on a straight pipe and BHAF.

 

[BHaden]

Mines about 2-2. 3 high almost every time. Its a little closer when I tow a full tanks worth but only a little. Like 1. 6-1. 8 high.

 

[mlewis73]

Mine is about 2. 5 mpg high. Wish it were more accurate. I guess 19. 4mpg is being too optimistic for a G56'd Ram 3500 dually...



Best use for the mileage display is as a tool to judge trends...

 

[testraub]

The computer shows "NOW" not overall from I can see. I "avg" 14. 2 to 14. 5 going to work and back Get out on the Hwy 19. 5to 20. 2 Based on tank and driving it's a toss up.

 

[2broke2smoke]

if it shows now then why doesn't it read 0 at an idle ...



ours is always 2 mpg too optimistic also

 

[Bernie]

Average

No, I'm pretty sure it reads the average since the last time you reset it:

-Labeled average

-Doesn't change (dramatically) with WOT acceleration or coasting down long hills

 

[phloop]

On my new '06 I only have a few miles on it but so far the overhead is about 3 miles over reality into fantasy land. I filled up today, the overhead read 16. 6 and hand calc. was 13. 860.



On my '04. 5 before the reflash both the overhead and hand calc. was really close, within less than a half mpg. After having the "mileage" reflash, the overhead read about 2-3 miles more while the real mileage stayed the same.



As with everything else that Dodge has done for "and in" our interest, looks good from the outside.

 

[Bernie]

Is it the 06s?

I'm seeing a trend in the posts. The 06s seem to all read about 2 MPG optimistic. the only post that read lower was from an 04, and it was only off about . 3 MPG.



This must be how Dodge makes its customers feel good about the economy of the truck. Lie to them.



So, what I really wanted to know and no one has said yet is technically how the calculations are done, and how do I correct the inaccuracy?

 

[AGrubbs]

viscosity

Mine usually reads 1 to 2 MPG higher than what I calculate, in the summer, and reads lower in the winter. I suspect that all it does is calculate the amount of "on" time for the injector, the size of the 'holes' and the fuel pressure of a standard fuel to come up with an amount of fuel that is flowing.



Thinner or thicker fuel (summer/winter blend) would make a difference in the readout then. Also any variations in how your high psi fuel sensor is calibrated would make a difference between one truck and another... Also if there is a fuel temp sensor, it might be used to calculate the viscosity, and it could be off a bit...



Lots of variables... I just use it as a relative indication of how I am doing driving for MPG's. . not an absolute real world number. .





Drive ON!

 

[Sandyneck]

My overhead is usually off by about . 5 mpg on the conservative side. I always hand calc the milage and I consistanly get from . 5 to. 75 mpg better than what the overhead reads. I usually wait until the tank is below the 1/4 mark before I fill it as well.

 

[Grayhackle]

My computer is consistently 2-4 mpg lower than calculated average.

So consistent, I can guess the calculated mpg.

 

[DieselFiend]

Mine ALWAYS reads at least 2 mpg high. I took it to the dealer and got a reflash for it and didn't seem to help out. I can't remember where I read this but I think the computer only averages for the last 60 miles or so, not since the last time you reset it. That would totally explain why mine consistantly reads 2 high. I'm usually easier on the pedal when the tank is below half.

 

[wyosteve]

Add another to the low side. It's always 1. 5 to 2. 0 lower than hand calc.

 

[rweis]

From an '02



Reads 21. 7 Actual 16. 7 ~ 30% higher than actual.



Injectors DD2 (80hp +-) or ~ 30% more than OEM.



I figuer it thinks averything is stock.



If you take what ever upgrades you have for hp as a % of stock, is your overhead about right?



Just an idea,



Bob Weis

 

[RBollengier]

Mine is closer when all the driving is about the same, reads about 1. 5-2 high. The higher it indicates the more it's off, I don't think it can measure small fuel flows well. When I tow it is much closer, less then one high.



I have also read that it averages the last 50-150 miles only. If I don't reset it for several tanks of fuel the "ave" still changes about the same rate as when I reset it at tank Fill-up when I tow after a long run of empty. The "ave" takes no longer to go from 19 to 16 when I tow my RC trailer if I don't reset it at fill-up or several tanks, about one trip to town and back (60miles).



I think it's a sliding average and not an absolute average and that's why it's off more when "all" the driving is not the same.

 

[2broke2smoke]

last weekend i drove from central wv to scranton pa ...

got 17. 8 on the way there and 17. 7 on the way back ... both hand calculated ... reset the overhead when i filled up, drove straight thru and filled up when i got to there ... both ways



both trips the overhead read 19. 8



just like the idiot gauges, dodge gives the sheeple out there what they want see and nobody questions anything

 

[Walker]

Mine's like Sandyneck's about . 5 on the low side is all. I'm an early 06 (Aug. 05), and I think the new software has the later date trucks reading up to 2-4 mpg on the high side. Know a couple of guys with new 06's that say they are getting 18-20 mpg running 75-80. But they never hand figure either. Bet they'd be suprised.

 

[Bob4x4]

Read tsb 08=02-06 fot explanation of how the avg mpg is done with the 06



Bob