I would like to know how accurate the overhead computer reading for mpg is. I see here that a lot of you get around 13-15 mpg in the city and around 17 -19 on the highway. According to my overhead reading, I get around 11-12 in the city and the best I have seen on the freeway with the cc set on 65, I was getting around 13-14 mpg. Does this sound right?
Thanks.

 

stock would be a bit low with the numbers....tire size looks to be the culprit, that is if you've not taken them into account with numbers.

 

The overhead lies. Never consistently either. Mines been plus or minus 0.5-3.0 the hand calculated average. I've yet to see it be the actual mileage.

 

On the highway mine is accurate. In town, not really at all, usually +/- 3mpg

HWY it is about .5 high, consistantly. Reset it when you fill up, and dont touch it. I have noticed that as you reset it with more fule gone, it gets more innacurate.

 

If your overhead trip computer MPG readout is wrong
that is a valuable clue.

It means your fuel pressure is either too high or low,
or fuel injectors are malfunctioning in some way.

Check the gallons side of the system
by writing down the gallons put in the tank
for 100 to 200 gallons,
and compare this to the overhead.

Check the miles traveled by either doing a test
against interstate mileposts,
or against a GPS.

See archived post below about how to have a tech adjust the PCM to get correct miles,
as many techs don't know the little details:
----
originally posted by grfraz
Augusta GA

Here is the recipe.

1) Have the tech plug his hand held into the OBD port and turn it on. A menu
screen comes up with a bunch of info. Select YES to go to next screen.

2) On this screen select menu item 1, "DRB III Standalone" to go to next screen.

3) On this screen select menu item 1, "1998-2005" to go to next screen.

4) On this screen select menu item 1, "All" to go to next screen.

5) On this screen select menu item 5, "Antilock Brakes" to go to next screen.

6) On this screen select menu item 9, "Miscellaneous" to go to next screen.

7) On this screen select menu item 2, "Set Pinion Factor" to go to next screen.

8) On this screen there are a several tire sizes and Other. Select menu item 8,
"Other" to go to next screen.

9) On this screen there are two items, an Axle Size, (9.25" on the 1500's and
10.5" on the 25/3500's) and Other. Select menu item 1, "9.25" Axle" (10.5 in
your case) to go to next screen.

10) You have now arrived at the point in this process where you will enter the
revolutions per mile for your tire size. Select the menu box for RPM. It will
indicate 400. Here is where the tech will change the revs per mile with either
the up or down arrow. It will take a while to go from 400 to 604 (for the BFG
315's) so be patient. As you are waiting on him to finish, take this time to let
him know that he shouldn't charge you more than say $20.00, especially since you
trained him on this procedure.

11) The last step is to select "OK" and the changes will be excepted, then the
hand held will display "Reprogramming Successful".

Good luck!

======
There is an online Java calculator to compute revolutions per mile at:

http://pw1.netcom.com/~sgalaba/tiresize.htm

 

Everybody complains about the overhead but mine seems to run pretty close to the pencil and paper numbers. Close enough, this ain't rocket science! Red

 

mine's pretty good too. it's +/- .5, though it is usually on the high side

 

Every two weeks I fill up and reset the over head. At the end of my two weeks my over head will read some where around 17.5 to 18.5. When I do the math I always have around 14.5 to 15.5.

 

Wether its accurate or not it still helps me figure out when im suckin back the fuel and when im doing fairly well, I try to find a mediun that way wether its accurate or not.

Its probably as accurate as a fish finder on a boat, you know where they show a black mark going across the screen which is supposed to be a fish. I once seen one for sale in a store and turned it on to see all the options and low and behold fish started swimming across the screen right there in the store lol.

 

as a rough rule of thumb
if your fuel pressure is too high above spec
your MPG readout on the trip computer will be too high
{because the PCM computer has adjusted back the milliseconds the fuel injectors squirt}

If your fuel injector tips are partially clogged with deposits
your MPG readout on the trip computer will be too low