Well like it says, I drive alot for work and have loged every fillup sense I got my truck in Dec. The past couple trips I havent reset the MPG calc and it is saying 18.7 which would be awsome if it were true. My acctual milage for the last 5 fillups has been 16.14. Is everyones off this much and did dodge use overhead mpg calculator to get there est mpg cause its pretty close, but in reallity its way off. ???

 

You can't really compare the overhead to the hand calculated MPG unless you reset the overhead after each fill-up.

If your overhead MPG shown on the trip computer
is way different than the hand calculated MPG using the odo miles and the gallons shown on the gas pump,
then the problem might be:
1. your fuel pump pressures are off specification
2. one or more of your fuel injectors are dribbling fuel when they should be shut off because crud deposits are keeping the 'pintle valve' at their tips from fully closing
3. you may not be filling the tank to the same level each time...it is easy to be off by 1-3 gallons which throws off the accuracy of your hand calculation.

Your overhead trip computer depends on your fuel pressure being very near to an exact psi of pressure and that the fuel injectors are working properly. If these two things are met, then the PCM computer keeps track of how much time the injectors are actually turned on and uses this to keep a running record of the gallons used over time. It is also possible (but unlikely) that the "clock" inside the PCM computer is off spec.

Even the hand calculated MPG is usually not trully accurate, because nearly every Chrysler vehicle I have ever owned or test driven is 4-8% high on the odometer miles. This makes you think the MPG is a little better than it trully is, and it allows Chrysler to get out of the warranty mileage 4-8% sooner.

In their defense Chrysler will say they do this to 'error on the side of caution' so that owners won't blame them for getting speeding tickets.

Since you are keeping a log (a great idea for spotting problems early)
compare the total gallons of five to ten fill-ups as read on the gas pumps to the total gallons of the overhead trip computer before passing judgement.

Official Mopar fuel injector cleaner is about $6 per bottle at the dealership and tested best on thousands of partially clogged injectors returned to Chrysler Tech center from dealerships.
Chevron Techron Plus is about $10
and Gumout Regane (the clear bottle only) is about $7.
There three are very good and putting one of each in the next few tanks of gasoline is the cheapest first step to making sure the fuel injectors have clean tips.

 

Everytime I checked mine, the computer was only about .3 mpgoff... I thought that might have something to do with just how full some pumps filled it up before it clicked off...maybe that little bit would make it off by that little

 

I get consist mpg with the hand calcs, and when I fill up I let the pump stop itself and then thats it, I know it can be different every time, but on average city/country driving is 14.3-15.4 and highway driving is 15.5-16.3. And I have rest the comp at time of fill up many times and city/country it give 16s and highway gives me 18s. Its pretty consistent. I cant see the injectors having any problems, being that I bought the truck brand new dec and have put all the miles on from 50-9300. Im just trying to figure it if the mpg are what I calculate which I would think is more accurate or what the comp says. Maybe its somewhere in between. As for the odometer being off I never did a tested lenght, but google and my odometer seem to pretty close on trips (not any justification either way). I guess Ill just keep up what Ive been doing hand mpg go into log book and comp mpg are there to make me feel better after writing in the log book.

 

Even if you use the same exact pump and handle every time you fill up, it is impossible for it to shut off with the same amount of fuel in the tank every time. The backflow shut off is set by the last person that tested it. There are too many variables to say that it will always shut off at the same tank level. Also the angle your truck is sitting will affect this. Even a low tire could cause you to get more or less fuel by using the backflow shut off as a fill indicator.

If you use the same formula to figure your MPG you will be able to spot a problem in your MPG very soon. But using the pumps reading and your trucks mileage for an exact MPG figure will only cause you headaches.

 

I know thats why I said "I know it can be different every time". I think the rest of what your saying is that using the same "formula" to calc mpg will show a change, but not show what you are acctually getting. My real problem is still why the pump/truck milage and the overhead mpg have a difference of over 2mpg and does anyone else see this in their truck?

 

I see it,but in the other direction. Using the computor,I get 14.8,when I did the hand calculation I averaged 16.4.beats the heck out of me!

 

Well really doesnt really matter that much anyways. Truck needs gas to move, truck needs to move to get to work, gas cost money, so you have to spend money to get to work to make money to get to work??? interesting.
New truck gets better mpg than my previous truck (5.9 2000 ram.) so cant complain that much.

 

Ck with your local dealer, I do know some cars or truck have a flash up date for this problem.. Or may not have an up date.

 

If you reset the overhead it is pretty close from my experience. If you don't and let it average itself out it can get tweaked. I went on a 400 mile trip the other day and average 16mpg. I filled up for work and didn't reset the overhead. I did a lot of in town driving and my mileage went down to 15.5 Now I know it's way lower than that but the computer was taking the accumalitve average. I think