It's the law of averages. The longer that you go without resetting, the more accurate it will (should) be. Ex: saturday, I went on a interstate trip of 150 miles each way. Prior, I filled to the neck at a station that is one mile from the on ramp, at about 12 miles or so into the trip, it climbed to 20.0 and leveled off. That was at around 57 MPH. 0nce speeds increased to 65, it slowly creeped down to 18.8 after 150 miles. After about 400 miles on the tank, I am showing 18.4, (towing) at exactly 1/2 tank. BTW- the needle did not move off the peg until literally 150 miles. I do not plan on resetting this tank for 3 tanks, to get a better gauge on how John Deere fuel treatment works, which I believe is stanadyne. I have always noticed that the deeper into a tank I get, the harder it is to get the overhead number to change, for better or worse.

 

Forget how you fill up your tank. It is really inconvenient--if not impossible to fill the same way every time. Just calculate your mpg and try to arrive at an average over several tanks. I don't go faster than 70 on Interstate and keep it at 65 on two lane and my tanks consistently average in the 20 range. Last winter I did 2400 miles round trip from Wyoming to Las Vegas and then to Yuma, AZ and averaged 20.4 hand calculated. The overhead was about .4 off on the low side. Recently, I think I have found a groove at about 63 mph and 1850 rpm. Although I don't have long term data, I think I may be able to get in the 22 range. It is to early to take that to the bank, though.

 

Yea thats right on the longer that you are into the tank the harder it is to get the readout to move much. The computer already has calculated 1/2 tank of fuel and is showing you the average of that. Now say that you are getting 18 mpg- - - - You then start to hammer it and it only goes down to 17.5- - - -Well that is because the larger amount of fuel that is being consumed, even though allot more than you were burning before, dosent offset the Half Tank that you have already burnt. Now if you start off hammering the truck until you only have 1/2 tank left and average 12mpg, good luck babying the thing back up to 18- - - -wont happen! The larger portion that has been burnt is now making the sipping less evident! Its all Math man- - - -and I really despize m$^h!

 

That's the "sweet spot" of the cummins...

 

I do not see how you guys can go 63 MPH or 60? Geez, I can barely go 75 on the interstate anymore. I usually go 70 on 2 lane and 75-80 on interstate. I am waiting for my truck to get to where it is broke in to see how much better the mileage gets to be.

 

I reset the overhead and the trip odometer at every fill up, never top it off. The overhead is consistantly within .2 shy of hand calculated figures. I've averaged 20-22mpg per tank all summer/fall, mileage typically drops to about 18 mpg over the winter.

 

My overhead is the same or pretty close if I do not top off my tank. But if I top it off then it (the overhead) shows to be 1.5 - 1.7 more than when I hand calculate it. I top it off everytime and it is still off by the same amount.