Ok this is going to sound really dumb but how does the JK computer figure the MPG? I very rarely reset mine but it seems that everytime I take a road trip it goes way up to quickly. Any clues??

 

im not sure how it calculates it.. however on a long roadtrip if I run a tank from full to empty it is very accurate for that tank of fuel... now if I am around town run a tank from full to half, and then go on a roadtrip the last 1/2 of the tank then the computer will be high for that complete tank... I know Cadillacs in the past always used the past 25-50 miles as its basis for the average as well as the DTE function.. so im guessing jeep does something similar to that..
-Christopher

 

I'll bet it's an integration function based off of the flow rate of fuel over the distance traveled. In fact, distance traveled is just an integration of velocity, how fast you are going in any point in time. It relatively easy to do in the PCM. It is a function of time, but what I don't know is the time constant used - does it integrate over 10 minutes? 15 maybe? Also, not sure how it initializes when you hit the reset button. Does it start at a fixed value and compute from there or does it start at the last saved value?

I did a little modeling of example automotive functions at a workshop a few years ago. We did two models, one for a simple cruise control and one for - you guessed it - MPG. These models were used to demonstrate some programming tools but were based on Ford's data inputs. This was pre CAN bus days, but I'm sure the data was pretty much the same.

Ah, back when I still remembered a little differential calculus. That skill is long gone!! (no - it's not like riding a bike)

 

I could not find anything specific relating to the JK, but other gauges I have seen use mass airflow to extrapolate fuel burn, since the fuel/air ratio is kept quite close.

 

If I had to guess I would say that it works on the same principal as the semis that I work on. I takes the amount of fuel in the tank by volume (gauge in tank) which the computer is calibrated for the tank size and shape, and fuel rate (rpm, engine load) then divides the miles ran since last reset by gallons used. That will give you your average miles per gallon. It will fluctuate more when first reset due to less miles to divide by. If you ran in the city and reset it would read alot lower til it averaged out (about 1/4 to 1/2 tank) It sounds like alot but is accually very simple.